Force of Nature
You can take the Barbarian, using your Strength in place of Dexterity for your Barbarian features.
Requirement: The only Barbarian subclass available to you is the Path of the Storm Herald. Additionally, at least half of your character levels (rounded up) must be in Barbarian.
[Example: A warrior returns from the wilds, changed. His body contains the literal essence of a storm, and only his indomitable physicality kept it contained.]
Spoiler: Force of Nature Analysis
I figured that since the Storm Herald has to focus on maximizing Constitution for the sake of their Saving Throws, and the fact that the Herald would not likely consider using a Bonus Action attack with a weapon, it is rather hard to cheese out spammed high damage Strength attacks if Dex was no longer needed. A Barbarian could choose to do something like GWM and max out Strength for high damage, but then they'd run into a problem where their subclass doesn't provide them anything for their playstyle. They'd have to invest heavily in both Strength and Constitution stats to get the most out of the subclass features, which becomes its own sort of MADness.
Additionally, Storm Herald is notoriously bad (I mean, 2 damage per round? Come on). You're forced to invest into Barbarian for this Option, so you can't just cheat and get Strength+Constitution version of Unarmored Defense. Still a buff, just not as big of a deal as you might initially think.
Force of Savagery
You can take the Barbarian, using Dexterity in place of Strength for your Barbarian features.
Requirement: The only Barbarian subclass available to you is the Path of the Berserker. Additionally, at least half of your character levels (rounded up) must be in Barbarian.
[Example: A savage has learned to tap into his primal instincts, and strikes at the speed of thought. However, doing so puts much strain on his body and mind.]
Spoiler: Force of Savagery Analysis
I find it's roughly balanced, due to the fact that the main benefit of the standard Berserker is to launch several high damage, Strength-based weapon attacks at the enemy, and the highest damage a Dexterity equivalent would be is a 1d8.
You effectively lose Reach and 1-2 damage for increased AC and ranged attacks. The redundancy with Two-Weapon fighting also makes it less of an appealing choice for optimizers who are hoping to break something in that direction.
The ranged attacks are also unusable for the Berserker's features, so all-in-all, it's not much better than the original, if at all, but it fits a niche that a lot of people like.
The idea is that it's a Barbarian that utilizes a Haste-like effect to move faster than his opponent, but it has similar drawbacks to the spell (in that it wears you out). Mechanically, you have more AC than any other Barbarian (as a fast Barbarian should) but you lose out on a decent bit of damage unless you're willing to Frenzy constantly, and you'll never outdamage a Strength version (although you'll probably live a lot longer).
Clergy of the People
You can take the Cleric, using your Charisma modifier in place of your Wisdom modifier for your Cleric features.
Requirement: You cannot take Warlock or Sorcerer levels.
[Example: A cleric chooses to live their life by aiding those in need, and to make the world a better place through the spoken word of the people]
Spoiler: Clergy of the People Analysis
You're going to hear this a LOT: Don't allow the Sorcerer and Warlock to multiclass with others. This is because they change how spellcasting works, to make it more versatile. Other classes, like the Cleric, can't use all of their features at the same time, but the Sorcerer and Warlock CAN (as their biggest features are Metamagics and Short-Rest Spell slot recharging). As an example, mixing the Cleric with those classes ends up with a high-AC, high-Concentration caster with too many tools and too much versatility. A Cleric/Sorcerer/Warlock could do anything, solve any problem, and be tanky to boot.
With these restrictions, a Bard/Cleric might have some slightly higher AC, but ends up only being able to cast as at the same combat level of a lower level Cleric. A Cleric/Paladin has a lot of redundant proficiencies. There's just not a lot you can break with these restrictions.
Disciple of Oghma
You can take the Knowledge or Arcana Cleric, using your Intelligence modifier in place of your Wisdom modifier for your Cleric features.
Requirement: You cannot have levels into the War Mage Wizard subclasses. Additionally, you cannot have more Wizard levels than you have Cleric levels.
[Example: A historian, honoring their deity through practice of their scriptures rather than through prayer.]
Spoiler: Disciple of Ohgma Analysis
The War Mage is a generic murder-hobo mage subclass, and I want to remove any incentive for pure-combat builds from these changes. Any combat Wizard with medium/heavy armor proficiencies and more level 1 spell slots is a recipe for disaster, so I find it's best to focus on the Cleric side of things (which deals considerably less damage).
Way of the Closed Fist:
You can take the Monk, using your Strength modifier in place of your Dexterity modifier for your Monk features.
Requirement: The only Monk subclass available to you is the Way of the Open Palm.
[Example: A troubled youth learns to harness his strength for good, channeling his forceful nature through his fists.
Spoiler: Way of the Closed Fist Analysis
A bit odd, I know, but the Open Hand relies really heavily on Wisdom compared to the other melee-oriented Monks. Your damage output is a bit higher by mixing in with Barbarian, but your Ki point reserve takes a hit. With no Wisdom, you deal the same damage as a Barbarian with a Monk dip, and having Wisdom means that your Strength/Constitution is going to be lower (meaning either less damage/hits, or less HP that stacks with Rage).
While the Barbarian/Monk combination does seem strong compared to a normal Barbarian, consider the fact that Rage works best with a high HP pool, and the Monk gains 2 less HP per level than the Barbarian. Leveling in this way means that you're able to absorb about 3-4 less damage per level than the classic Barbarian, making this an even less effective choice.
Lastly, you also weaken the Monk's Evasion ability by changing it to Strength, a much less common and less threatening Saving Throw.
Way of the Disciplined Eye
You can take the Monk, using your Intelligence modifier in place of your Wisdom modifier for your Monk features.
Requirement: The only Monk subclasses available to you are the Ways of the Sun Soul, Four Elements, and the Long Death. Additionally, you cannot have more Wizard levels than you have Monk levels.
[Example: A reclusive student learns the forbidden arts of his temple after much time spent in the library, and seeks to hone his newfound knowledge with practice.]
Spoiler: Way of the Disciplined Eye Analysis
Make sure to not allow a Wizard to cheat his way into high AC without some kind of cost. In this case, we're talking about Unarmored Defense, which can completely go out of hand when considering something like Bladesinger's AC bonus.
It can be a concern of a Monk dipping a few levels into Wizard, which provides things like Shield and Absorb Elements, but this does mean a smaller HP pool. The War Mage is a solid choice, but the Reaction benefit really competes with the Monk's existing Reaction uses (Deflect Missiles, Slowfall) which are already very powerful choices. You are not gaining the ability to Deflect Missiles AND gain +2 AC AND gain Shield, only the option to do one of the three, and that's perfectly reasonable.
A Monk built this way cannot go fully Intelligence and abuse Bladesinger or Wizard levels, due to the fact that effects like Shillelagh are only possible using Charisma (via Tome Warlock) or Wisdom. Intelligence has no method to attack with a weapon, and so a Dexterity-less Monk will have VERY weak attacks compared to a balanced one. A fair trade, considering the spellcasting and AC benefits.
Oath of Superiority
You can take the Paladin, using your Intelligence modifier in place of your Charisma modifier for your Paladin features.
Requirement: The only Paladin subclass available to you is the Oath of Conquest. Additionally, you cannot have more Wizard levels than your Paladin levels.
[Example: A dark knight uses his superior intellect to dominate those with weaker minds than him.]
Spoiler: Oath of Superiority Analysis
Make sure that Wizards can't get cheatyface AC from a low level dip. A main-Paladin can dip into Wizard for Shield and Absorb Elements, but this is already doable with Sorcerer, and Sorcerer can cheat out Twin Spell Booming Blade, so it's still better than the Paladin/Wizard Hybrid.
Admittedly, the restriction for Conquest is a narrative choice, but no other Paladin Oath really seemed to justify a change to Intelligence. I added this one mostly due to popular request.
Oath of the Old Ways
You can take the Paladin, using your Wisdom modifier in place of your Charisma modifier for your Paladin features.
Requirement: The only Paladin subclass available to you is the Oath of the Ancients. Additionally, you cannot have more Druid levels than your Paladin levels.
[Example: An orphan, raised by denizens of the forests, grows to be its solitary protector.]
Spoiler: Oath of the Old Ways Analysis
Going through a checklist reveals that nothing is broken by this option:
Clerics already have access to armor and healing of a Paladin. Paladin provides some burst damage potential, but at the cost of a lot of redundant abilities (like inefficient healing via Lay on Hands vs. Healing Word, or redundant armor/weapon proficiencies).
Druids gain some armor, and have decent spellcasting, but their emphasis on Concentration doesn't do well with a primarily melee multiclass. Moon Druids get a minor buff, but that armor won't be usable in this combination. Similar to Clerics, but while the Druid has more close-ranged damage spells than Clerics, they rely on Concentration too much for a Paladin/Druid to rely on them too much.
Rangers get heavy armor, I guess? Not much else to gain here.
Monks get Divine Smite, but not much else.
In all of these circumstances, a multiclass will generally cause the Paladin to lose Paladin levels and delay those powerful Paladin features, so it all seems fine to me. Especially when the Paladin+Sorcerer Twinned Booming Blade or Paladin+Warlock Smite Spam options are much more explosive than anything a Wisdom Paladin could get.
You can take the Sorcerer, using your Wisdom modifier in place of your Charisma modifier for your Sorcerer features.
Requirement: You cannot take levels into Cleric or Druid. You must be able to cast Detect Magic.
[Example: A powerful mage, aware of all forms of power around him, can feel nearby magic and bend it gracefully.]
Spoiler: Sorcerous Medium Analysis
Cleric and Druids have some solid armor and a lot of versatility as prepared casters. Don't exaggerate that by allowing them to multiclass with Sorcerer. Other than giving Monks and Rangers Shield, there's not anything scary by this choice. Those classes have access to a lot of great Concentration spells, and those Concentration spells are easily abusable with Metamagics.
Detect Magic was added as a requirement to lower the overall power creep of mixing the Sorcerer with a Martial class. Now you have slightly fewer murderhobo shenanigans.
You can take the Sorcerer, using your Constitution modifier in place of your Charisma modifier for your Sorcerer features.
Requirement: Between 1/5 and 1/2 of all of your Class levels must be in Sorcerer.
[Example: A warrior receives a gift from a powerful benefactor, aligning his fate with a new purpose and power.]
Spoiler: Sorcerous Adept Analysis
You can mix-and-match this in a lot of different ways-kinda the point-but there's not too many means of breaking it. In most situations, you're better off just taking a level or two into the normal choice for a particular build, like Eldritch Knight + Wizard, or Monk + Druid. The forced 1/5-1/2 ratio means you're unable to abuse Sorcerer spell slots to their maximum capability, and you're unable to leave the level where it lies.
As a result, you delay Extra Attack, higher level spells, and a whole plethora of benefits to...gain Shield? Move as you cast spells? Enhance your healing slightly?
Even something that maxes out Constitution, like a Barbarian, might reconsider when he has to wait until level 7 to get Extra Attack.
Chosen of the Forbidden Blade
You can take the Hexblade Warlock subclass, using your Wisdom modifier in place of your Charisma modifier for your Warlock features.
Requirement: Half of your total character levels (rounded up) must be in Warlock.
[Example: A sentinel in a temple maintains a corrupted spirit blade, until it breaks. The spirit now inhabits him and grants him dark powers.]
Spoiler: Chosen of the Forbidden Blade
You want to avoid letting characters dip into Warlock easily, and Hexblade is the biggest example of what a problem it can create, with access to Shield and all.
My solution is to make Warlocks a primary class, which removes any kind of low-level magic cheese. Warlocks have so many build options that players aren't really forced into any one playstyle by doing so.
Chosen of the Fey
You can take the Archfey Warlock subclass, using your Wisdom modifier in place of your Charisma modifier for your Warlock features.
Requirement: You cannot have more Ranger levels than you do Warlock.
[Example: A guardian of the wilds finds a benefactor for their cause.]
Spoiler: Chosen of the Fey
Ranger gets a lot out of a mobile caster like this, not to mention that Ranger provides all the proficiencies a Warlock would need, so I made the restriction specific to them.
Monks would get Absorb Elements, but otherwise wouldn't be able to abuse the spellcasting.
Druids focus heavily on Concentration, and other than that, wouldn't become that much more effective if they were allowed to have Short-Rest spell slot recharging (which is what the Land Druid does anyway).
All-in-all, not much of a problem in any scenario.
Chosen of Death's Defiant
You can take the Undying Warlock subclass, using your Constitution modifier in place of your Charisma modifier for your Warlock features.
Requirement: Half of your character levels (rounded up) must be in Warlock.
[Example: A hero stays alive by bonding his soul to an immortal being who is impressed by the hero's stubbornness to die. Invigorated, the hero now relies on this bond to stay a hero.]
Spoiler: Chosen of Death's Defiant
One of my favorites, it works well with multiclassing into a martial class, but it doesn't allow you to dip for an easily overpowered Eldritch Blast.
It's a minor boost to the power level of the Undying Patron, but it's not like it's going to cause people to take it in swarms. You're still limited in your damage abilities, and you can't invest a whole lot into martial classes. You could take a level into Barbarian and rock some Rage + Armor of Agathys, but that's an underplayed scenario with a lot of cool things going for it (as well as a lot of self-harming effects, like Rage and casting). This was an intentional buff, but nothing I can come up with really makes it overpowered.
Chosen of Infinite Truths:
You can take the Great Old One Warlock subclass, using your Intelligence modifier in place of your Charisma modifier for your Warlock features.
Requirement: Half of your character levels (rounded up) must be in Warlock.
[Example: A Mage, willing to do whatever it takes for more knowledge and power, sells his soul to the highest bidder and spies on the mortal world.]
Spoiler: Chosen of Infinite Truths
Once again, be careful about letting other classes easily dip into Warlock.
All the existing intelligence characters may have mixed feelings about dipping into a non-combat oriented Warlock subclass that you're forced to make your primary class, but that's the cost of Short Rest Spell Slots.
You can take levels into Wizard, using your Charisma modifier in place of your Intelligence modifier for your Wizard features.
Requirement: The only Wizard subclass available to you is the School of Enchantment. Additionally, you cannot take levels into Sorcerer or Paladin.
[Example: A magical con-artist learns that manipulation starts with practice before study, and combines both to reach his goals.]
Spoiler: Natural Enchanter
Blocked Paladin to avoid allowing Wizards access to cheap and easy AC, but also for a narrative reason (as this is an Enchanter who learned to manipulate out of habit). Sorcerer is blocked, mostly because of the fact that Metamagic is very powerful with single target Enchantment spells.
Warlock isn't blocked for once, due to the fact that the Enchanter has a lot more options to work with than spamming Tasha's Hideous Laughter, or other Concentration spells, and doesn't really get much out of the Warlock. You let the Wizard be able to cast Shield more often, but the Enchanter might as well have just invested into more Wizard levels.
You can take levels into Wizard, using your Wisdom modifier in place of your Intelligence modifier for your Wizard features.
Requirement: The only Wizard subclass available to you is the School of Divination. Additionally, you cannot have levels of Cleric.
[Example: A psychic has learned to tap into subtle signs of the future world around them, constantly aware of that which is and will be.]
Spoiler: Natural Diviner Analysis
You want to avoid letting Clerics and Wizards merge willy nilly, due to how powerful Wizards can be if they're allowed to maintain Concentration for too long when considering the Cleric's AC. Cleric dipping into Wizard is calling for unwanted Murderhobo abuse with Shield and Absorb Elements on something like Tempest Cleric that would outshine almost any other gish build, so it's best to just not let Clerics and Wizards mingle.
Other Wisdom classes, like Monks, Druids or Rangers, don't provide ludicrous levels of AC for low investments, nor do they have many things that enhance the Wizard's ability to cast spells. If anything, you might see those classes dip a level into Wizard for Shield or something, but most of them generally want to avoid direct combat and won't get the extreme levels of mitigation that a Fighter or Paladin with Shield would get.