This is meant to be a list of minor and a few major tweaks to D&D 3.5 that may or may not ever see actual play, but since I don’t really have the ability to playtest it at the moment (outside running both sides of a combat) I figured I should at least post the whole thing online and get some feedback. The main problem I see with it is complexity but I don't think it's that much more complex compared to the base game. How much of this is actually viable?
Note: I don't have the post count to use links, so anything I reference has to be searched up yourself.

Spoiler: Class Tiers
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So, the biggest change by far is the rebalancing of classes. Given that going through each class and adjusting it piecemeal, or originating new classes, is both too much work and too much to remember, I went for a more brute force method.
I simply banned all Tier 1, Tier 2, and Tier 6 classes, split the remaining 3 tiers into 5 tiers, and gave lower tiers an accelerated acquisition of feats and ability score increases. The idea is to create a balance around Tier 3, with lower tier classes making up for their more linear class features with a less linear Effective Feat Level (EFL), which would give feats at first level and every 3rd level and ability score increases at every 4th level. Multiclassing characters add up their EFL from each class to determine their total.
Spoiler: Classes by Tier
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This is heavily based on the class tier update of 2019 thread on GITP, with a few adjustments. This is with the assumption that all build options (feats, ACFs, spell lists, etc.) from all 1st and 2nd party sources are available unless otherwise specified. This only includes base classes, the tier of prestige classes is determined on a case by case basis if the issue comes up.
Spoiler: Tier 3
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Shugenja (CD), Bard (PH), Trickster Variant Spellthief* (CA, Dr 353), Jester (DrC), Swordsage (ToB), Totemist (MoI), Crusader (ToB), Warmage (CA), Binder **(ToM), Warlock (CA), Warblade (ToB), Dragonfire Adept (DrM)
Spoiler: Tier 3.5
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Wild Shape Ranger Variant (UA), Duskblade (PH2), Factotum (Ds), Psychic Warrior (EPH), Lurk (CP), Psychic Rogue (web), Wild Monk (Dr 324), Incarnate (MoI)
Spoiler: Tier 4
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Shadowcaster (ToM), Rogue (PH), Barbarian (PH), Generic Expert (UA), Generic Warrior (UA), Divine Mind Ectopic Ally Variant (CP, web), Healer*** (MH), Master**** (WotL), Scout (CA), Spellthief^ (CA), Paladin (PH), Adept (DMG), Ranger^^ (PH)
Spoiler: Tier 4.5
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Ninja (CA), Savant (DrC), Fighter (PH), Marshal (MH), Sohei (OA, Dr 318), Hexblade (CW), Divine Mind^^^ (CP)
Spoiler: Tier 5
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Monk^^^^ (PH), Battledancer (DrC), Mountebank (DrC), Samurai (OA, Dr 318), Dragon Shaman (PH2), Magewright (ECS), Swashbuckler (CW), Mariner (LotT), Knight (PH2), Noble (DCS), Soulborn (MoI), Soulknife (EPH)
Spoiler: footnotes
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*Has no access to the bard spell list; **Has no access to Zceryll; ***No BoED spells or variant companions; ****Cohorts and Followers must also be Masters; ^Trickster Variant is Tier 3; ^^No Mystic Ranger, Wild Shape variant is Tier 3.5; ^^^Ectopic Ally variant is Tier 4; ^^^^Wild Monk variant is Tier 3.5
Spoiler: Full EFL by Class Tier Table
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Level 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Tier 3 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Tier 3.5 1 2 3 4 6 7 8 10 11 13 14 16 17 19 20 22 23 25 27 28
Tier 4 1 2 3 5 7 9 11 13 15 17 19 21 24 26 28 31 33 36 38 41
Tier 4.5 1 2 4 6 8 11 14 16 19 22 26 29 32 36 39 43 47 50 54 58
Tier 5 1 2 5 7 10 14 17 21 25 30 34 39 44 49 55 60 66 72 78 84

Spoiler: Builds
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Spoiler: LA Point Buy system and Races
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LA +0 races get 25 point buy, LA +1 get 18 point buy, LA +2 get 10 point buy, and LA +3 get 0 point buy. Additionally, a lot of races and templates have LA adjusted and/or RHD removed and/or stats adjusted in order to give more variety, with the available races adjusted to the campaign in question. The list of changed races and templates is too long to list here though.
Spoiler: Miscellaneous
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HP are max at 1st level and half at subsequent levels. No multiclassing XP penalty. Fractional base save and base attack bonuses (but the +2 bonus for a good save stacks). Prestige classes are available and have a tier on a case by case basis. XP is earned based on the Level Independent XP variant from UA.
Spoiler: Favored Classes
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Gaining a level in a favored class gives +1 hp and +1 skill point (or +4 at 1st level) and a new feat Additional Favored Class adds any base class as a favored class. Races with favored classes that are now banned are switched from Cleric to Adept, from Druid to Ranger, from Sorcerer to Shugenja, and from Wizard to Magewright.
Spoiler: Incarnum changes
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The Open ___ Chakra feats from MoI work like Martial Study. Characters without meldshaper levels get +1 to their max number of chakra binds, whereas characters with meldshaper levels don’t. A new feat, Extra Chakra Bind, requires one meldshaper level, increases the maximum chakra bind limit by one, and can be taken multiple times. Another new feat, Practiced Meldshaper, improves meldshaper level to character level.

Spoiler: Skills
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Spoiler: Miscellaneous
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Craft check results are in gp, not sp. Diplomacy uses the updated rules from the Alexandrian website, as does Tumble. Search checks cover a 15 ft by 15 ft area.
Spoiler: Stealth and Perception
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A creature gains +10 to Hide and Move Silently when not moving, though it still takes a move action. A creature gets a free Hide and Move Silently check if possible at the end of an attacking, running, or charging action with a -20 penalty. Actively making a Spot check or Listen check is a move action and targets the previous Hide or Move Silently check.

Spoiler: Combat
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Spoiler: Natural 1s and 20s
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Natural 1 on a save doesn’t risk damaging items. Rolling a natural 1 or natural 20 on a save or attack isn’t an automatic failure or success and is instead treated as a -10 or 30 respectively.
Spoiler: Attacking
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Iterative attacks are always at -5. e.g. a BAB of +20 becomes +20/+15/+15/+15 on a full attack. Having a BAB of +8 allows an iterative attack on a standard action attack, and having a BAB of +15 allows two such attacks. Extra attacks from natural weapons, TWF, etc. don’t apply. Having a BAB of +12 allows a single attack as a swift action.
Spoiler: Critical Hits
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Effects that improve critical hit ranges stack unless they’re from the same type of source. So multiple keen edge spells wouldn’t stack but one would stack with a keen weapon enhancement or the Improved Critical feat.
Spoiler: Injury and Death
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Death is at a number of negative hp equal to twice the creature’s CON score. A living creature is staggered/disabled from 0 hp until their negative CON score, then dying from their negative CON until death.
Spoiler: Massive Damage
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Massive damage happens when a character takes more damage from a single attack than their massive damage threshold, which is 25 plus or minus 5 per size category larger or smaller than medium plus 2 per HD. The subsequent Fort save is at DC 15 plus 1 per 5 hp past the threshold, and failing the save drops them to 0 hp (and are therefore staggered/disabled).
Spoiler: Armor Damage Conversion (from UA)
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An armor bonus to AC also converts some lethal damage to nonlethal damage and negates some nonlethal damage. The amount converted or negated is a maximum of the armor bonus in damage per attack. Any damage not susceptible to damage reduction (e.g. energy damage, and damage from spells, SLAs, and supernatural abilities) is also not subject to damage reduction.
Spoiler: Surprise Round
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A creature can ready a standard action for the surprise round without knowing when it will happen, essentially spending a standard action every round to ready that action. It must use this action, even if the initial situation makes the action disadvantageous. Delaying in the surprise round until the beginning of the first normal round in order to get a full round of actions is allowed, but gets an initiative result no higher than 20 + their initiative modifier.
Spoiler: Parrying (adapted from Dr 301)
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A new special attack, Parry, is specified as follows. As an AoO 1/round, can attempt to cancel a melee attack that has already hit (but before damage is rolled). Roll a melee attack roll against the result of the attack that hit, with a +4 bonus/penalty per size category larger/smaller than the attacking creature and a -4 penalty for using a natural attack. Cannot be used when wearing Medium or Heavy armor (or being encumbered), when unarmed, or when denied a DEX bonus to AC. Parrying provokes a disarm attempt that doesn’t provoke an AoO or allow a disarm attempt in return. When used against a critical hit works on the critical confirmation roll but not the initial attack.
Spoiler: Parrying Feats
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Armored Fencer (Heavy): DEX 13, INT 13, Combat Expertise, Armor Proficiency (Heavy), Armored Fencer (Medium): Can parry in heavy armor
Armored Fencer (Medium): DEX 13, INT 13, Combat Expertise, Armor Proficiency (Medium): Can parry in medium armor
Crushing Defense: STR 13, DEX 13, INT 13, Combat Expertise, Power Attack, Improved Sunder, BAB +4: When successfully parrying a manufactured weapon, get a free sunder attempt that doesn’t provoke AoO
Guarded Defense: DEX 13, INT 13, Combat Expertise, Improved Disarm, BAB +4: Parrying does not provoke a disarm attempt
Improved Parry: DEX 13, INT 13, Combat Expertise, Combat Reflexes: Can make more than one parry attempt per round, but each one still uses up an AoO
Incredible Parry: DEX 13, INT 13, Combat Expertise, BAB +6: +4 to Parry attempts, can be taken more than once
Protective Parry: DEX 13, INT 13, Combat Expertise: Whenever a creature you threaten is attacked with a melee attack, can attempt to parry that attack (still uses up an AoO)
Steel Skin: DEX 13, INT 13, Combat Expertise, BAB +4; Can parry with an unarmed strike with a -4 penalty (note that Monk and Battledancer treat unarmed strikes as natural weapons and thus don’t need this feat)
Spoiler: Conditions
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Fear now works as stacking penalties. Shaken still gives -2 to attacks, saves, and checks, but Frightened now gives a -4 to these checks instead of making the target run away, and Panicked now gives a -6. Weapons held in a locked gauntlet aren’t dropped when stunned, and magic items that use the hands slot can usually be made with a locked gauntlet.
Spoiler: Engulf Clarification
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Engulf now works like Trample. It’s a full round action that allows the creature to move up to twice its speed and engulf any creature of the appropriate size whose entire space it move over. The AoO mentioned is the same as, not in addition to, the oe provoked by moving into an opponent’s space, and it doesn’t have the -4 penalty the one vs Trample has.

Spoiler: Adventuring
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Spoiler: Encumbrance by Stone
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Items are stored in bundles of 20 pounds or less, with the size of light, medium, and heavy loads determined by the number of pounds divided by 10 rounded up. So more weight carried overall in exchange for the convenience of being able to play around with weight without calculating item by item.
Spoiler: Passive Training
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For verisimilitude, all intelligent creatures automatically earn a minimum of 10 XP per day spent actively practicing their skills. This explains how NPCs gain levels or make magic items without encounter XP or story XP. Since the Level Independent XP table has higher thresholds, this XP is almost useless to players but at least gives something for whenever one PC makes an item and the others have nothing to do for a week.
Spoiler: Repeated Encounters in a Day
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The first encounter in a day has only 50% XP, the second 75% XP, the third 100%, the fourth 125%, etc. This is a solution for the problem of the 15 minute adventuring day by providing an incentive to have multiple encounters and conserve resources rather than using all resources then waiting for the next day to arrive.

Spoiler: Magic
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Spoiler: Alter Self, Polymorph, and Metamorphosis
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These spells now follow the Polymorphed template as described by the Giant on this forum, as does Wild Shape. Any classes that did gain Polymorph now gain the Sor/Wiz replacement spells at the appropriate spell level. Alter Self remains as normal but is now restricted to the Polymorphed template as well as the spell description. Metamorphosis follows the Polymorphed template but isn’t split up. As stated below, Assume Supernatural Ability and Metamorphic Transfer are banned.
Spoiler: Arcane Disciple
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This feat only works with prepared arcane casters. When taking it multiple times, must choose domains from the same deity but the once per day per spell level usage is per domain, not total.
Spoiler: Spellcasting Monsters
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Almost all monsters with innate spellcasting ability (not SLAs) have that removed and their CR dropped accordingly.
Spoiler: Metamagic Changes (from UA)
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When using a Metamagic feat, it is applied spontaneously, doesn’t increase the casting time for spontaneous casters, and doesn’t increase the spell’s level. However, the caster must have access to the spell level it would normally take. Each metamagic feat can be used 3/day and can be taken another time to give three more uses. Using a metamagic feat in weird ways, like Divine Metamagic and Metamagic Spell Trigger, function normally and don’t count against this limit. Heighten Spell raises the effective spell by the maximum amount. Using more than metamagic feat or the same feat more than once expends multiple uses and requires access to the total adjusted spell level. The Sudden Metamagic feats work as normal. Unlike standard metamagic feats, these only work 1/day, can be applied to any spell level, can be taken only once, and have and count as different prerequisites.

Spoiler: Magic Items
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Any item in the MIC used the version there rather than any alternative version. The MIC rules for adding common magic item effects and for combining magic item effects with the same body slot apply. Magic and mundane items that are uncommon or difficult to make (either in the region or everywhere) have a higher effective price for determining whether a community of a certain size has it available to sell.

Spoiler: Alignment
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Spoiler: Evil Acts
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Poison isn’t evil (unless it’s excessively painful), creating undead isn’t evil, killing someone for the sole purpose of turning them into undead is evil. Murdering someone to steal their stuff is more of a neutral than evil act depending on the context and how much a campaign has that as the core experience. Just trying to avoid the weird hypocrisies about what is and isn’t evil by focusing on intents and results rather than a [Good] or [Evil] tag.
Spoiler: Alignment Scale
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Alignments are both the normal nine and an ordered pair on the scale ([0, 20], [0, 20]). For the first number from 0 to 6 means Lawful, from 7 to 13 means Neutral, and from 14 to 20 means Chaotic. For the second number the same ranges mean Evil, Neutral, and Good. Basically, people can now go against their alignment from time to time and still stay within it as long as they follow it most of the time.
Spoiler: Working Definitions
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Spoiler: Good vs Evil
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Good creatures want everyone to benefit, neutral creatures want their particular group to benefit, and evil creatures want themselves and possibly a few others to benefit. This definition tries to allow for neutral creatures to commit violence without being explicitly evil and still allowing good PCs to fight them. This hopefully avoids the issue of “good humans/elves/whatever, evil orcs/goblins/kobolds” even though both sides periodically kill and oppress the other. In this system, they’re both just neutral societies.
Spoiler: Law vs Chaos
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Lawful creatures are risk averse and Chaotic creatures are risk friendly. This definition tries to avoid the issues of what actions are lawful being defined by an arbitrary authority and the unfortunate choice of many chaotic PCs to make trouble for its own sake (which is neither realistic nor enjoyable).

Spoiler: Banned
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Banned books are any 3.0 content, BoED, and any 3rd party content. Banned feats are Leadership, Greenbound Summoning, Augment Summoning, Corpse-Crafter, Natural Spell, Item Familiar, Extraordinary Artisan, Mercantile Background, Tashalatora, Troll-Blooded, Silver Tongue, Boomerang Daze, Arcane Thesis, Easy Metamagic, Practical Metamagic, Midnight Metamagic, Metamagic School Focus, Residual Magic, Rapid Metamagic, Metamagic Song, Assume Supernatural Ability, Metamorphic Transfer, Versatile Spellcaster, Dragon Cohort, Undead Leadership, Wild Cohort, and Craft Contingent Spell. Nightsticks are banned. Things that depend on unused rule systems (e.g. taint, defiling, insanity) are banned. Note that things may be added or even taken off this list with DM approval (e.g. a 3rd party prestige class the player really wants). Specific build options available in a campaign may depend on the region in which it takes place.