The 5d20 System

[HouseRules]

Instead of rolling a single d20, players would roll 5 of them. Many elements of the game needs to change to adjust for this difference.

BASIC Changes

1) Ability Score Modifier Changes
Primary Attribute Modifier = Ability Score, more than double standard
Secondary Attribute Modifier = Ability Score / 2, it's five better than standard
Tertiary Attribute Modifier = Ability Score / 3, better than standard if your ability score is < 32.
We need a clear distinction between the types of attributes.
Those Single Attribute Dependency Classes are not always better now.

2A) Armor Class Changes:
Armor Class needs a base of 52 (+42 as a conversion) to adjust for the shift of average.
We rename Armor Class to Physical Defense Bonus.
The 50% CI is 43 to 62 with an average of 52.5 compare to 5 to 15 with an average of 10.5.
The 90% CI is 30 to 74 compared to 2 to 19.
2B) BAB is changed to Full = 4, Partial = 3, Half = 2.
No more fractional Base Attack Bonus
2C) Damage Reduction will change to Damage Resistance
Each Type of Damage has a separate Damage Resistance.
2D) Spell Resistance replace by Magical Defense Bonus
Magical Defense Bonus = Spell Resistance x 5
Characters/Creatures without a Spell Resistance Stat automatically have Spell Resistance 11 or Magical Defense Bonus of 55.
Caster must always attempt to bypass Magical Defense Bonus to affect creatures.
Character/Creatures may opt to lower their Magical Defense Bonus, this is for healing spells to work correctly.

3A) Difficulty Class Changes
Like Armor Class, a +42 is needed to convert from the standard d20 to the 5d20 system.
3B) Skills is changed to Class = 3 Ranks, Cross-Class = 2 Ranks.
No more fractional Skills
3C) Saves is changed to Good = 3, Poor = 2.
No more fractional Saving Throws
Another category of Saving Throws: Parring Save. Any Melee Characters may attempt to parry missiles or spells aimed at them, or a corner adjacent to them for area of affect spells. Parry an individual target missiles or spells would redirect the spell away from the character, however, total range of the spell is taken into account, and the spell may fissile if the new target is too far. Parrying an area of effect missiles or spells would shift the center of the spell to the new its location, but like above, if the new location is too far, the spell would fissile out. For missiles, parrying distance travel does not take into account the total distance travel. Instead it takes half the throwing distance of the character. This is a strength based saving throw.

4) Critical Hit Changes:
Numbers are shifted up by 80, and that means that critical hits become so much rarer.
20-20/x becomes 00-00/x
19-20/x becomes 99-00/x
18-20/x becomes 98-00/x
17-20/x becomes 97-00/x
16-20/x becomes 96-00/x
15-20/x becomes 95-00/x
...
02-20/x becomes 82-00/x
No limit on the stacking of critical threat range increase, though standard multiplication rules apply, x2 + x2 = x3.
Your critical hits are not relevant until you could expand your critical threat range to beyond 23 threat range (78-00/x) which gives 5% change to critical hit.

How do I go about with changing the other Sub-Systems to match?

[aimlessPolymath]

All forms of bonus-related effects get dramatically rescaled, obviously- a spell that gave +1 to something has to be rescaled significantly.

I'm not really sure what this change is solving within 3.5 that couldn't be done much easier with 3d6. What game design functions are served by this?

[noob]

I think the point is to spend lots of $ in twenty sided dices.

[Composer99]

If you want a range of numbers up to 100, why not use percentile dice?

[HouseRules]

The behavior is different, and the scaling is not linear.

The behavior is 5d4 for some aspects, 5d5 for others, and 5d6 for yet others if the scaling is linear. That is clearly different from 3d6.
Only Inherent Spell Resistance is scaled, but Spell Resistance by equipment is not scaled up.
Not sure how to scale magic to weaken it, but those broken spells need correction.
Skills are clearly not scaled linearly (2 = 1/2 of 3).

Martial Classes are weaken by the decrease in critical hits, so we return to 3.0 version where critical threat range has no limit to stacking, but that is not enough to close the new gap.

Percentile Dice is linear. 5d20 has a range of 96 values not 100. It could have 101 range by counting dice falling off the table as zero.

Chainmail use 2d6. D&D use d20. No change in static values (Armor Class) at all (besides switching from ascending to descending). 2d6 cannot roll 13-20, so Chainmail Rules have a huge disadvantage. That's why Gary Gygax never return to the Chainmail Rules (2d6) and used the Alternate Combat System (d20).

[noob]

If you want a range of numbers up to 100, why not use percentile dice?

A percentile dice will give equal weight to the borders and the middle of the range of values.
That person does not wants that.