House Rule: Extremely limited stacking, Bounded Accuracy on Steroids

[jodyjohnson]

The advantage/disadvantage system for modifiers takes care of a lot of the fiddly situation modifiers.

The class features, magic items, and spells leave a bunch in, potentially enabling players and DMs to push their bonuses beyond Bounded Accuracy.

House Rule: the base calculations for a game mechanic work as written, however anything that adds to that only takes the best positive modifier and worst negative penalty.

Specifics and Categories:
Shield spell and Shield item grant the same bonus - a magical shield bonus increases the value of the shield modifier (+5 bonus from shield spell is the same as a Magical Shield +3). The +1 bonus from Dual Weapon feat is a Shield bonus. Any ability which provides a bonus to AC by Parrying counts as a Shield bonus.

Other abilities or bonuses that increase AC don't stack beyond the AC formulas (AC=## - including Barkskin, AC=##+Dex (Max 2), AC=##+Dex, AC=10+Dex+Con, AC=Dex+Wis).
The Defensive Fighting style (+1), Magical Armor Enhancement (+1 to +3), Ring of Protection (+1), and other AC bonuses do not stack.

Attack bonuses are defined as a Base of: d20+Proficiency (or not) + Stat bonus. Any bonuses beyond that do not stack. Only the best bonus of the following applies per attack: Archery Style (+2), Bless (+1d4), Magical weapon enhancement (+1 to +3), Magical ammunition enhancement (+1 to +3), War Domain Channel Divinity +10, Bardic Inspiration (+1d6 to +1d12), etc.

Saving Throws: d20+Proficiency (or not) + Stat modifier. Resistance (+1d4), Bless (+1d4), Cloak of Protection (+1), Paladin Aura (+Charisma), Bardic Inspiration (+1d6 up to +1d12), etc.

Ability Checks: d20+Proficiency (or not) + Stat modifier. Guidance (+1d4), Expertise (+2 to +6), Bardic Inspiration (+1d6 up to +1d12), etc.

Damage bonuses: go nuts.

Not a house rule appropriate for every player or every group, but I think if chasing bonuses is having an adverse effect on your game this is a way to go.

 

[Sadrik]

This is a really good thought experiment.

I think you can break it down further and you may not be considering a few things.

So there are a few different types of rolls in the game:
You have skill checks, attack rolls, saving throws, and damage rolls. Those are the main ones any way.

For this, let us not consider opposed rolls, those are whacky and ill conceived anyway.

For three of the four rolls they essentially work the same d20 + mod vs a DC.

You addressed the d20 side of that equation without really addressing the DC side of that equation other than AC.

So here is my analysis,
If you were to go with only the highest bonus, the top level of bonus might be +10 but that would be tremendous, a 30 stat, stupendously powerful armor (the armor list needs to be reevaluated), and any other possibilities. Most likely I would think that somewhere around +6 would be the natural max to any roll.
If d20 will always be d20 + modifier (modifier can come from any one source: ability, proficiency, magic, spell, item, etc.)
For all instances when the roll is opposed (such as an attack roll, hiding, or a save) the DC should be 10 + highest modifier (modifier can come from any one source: ability, proficiency, magic, spell, item, etc.).
When the DC is is based purely on the difficulty it should not be the range presented in the books. With +6 being the typical top level modifier. The below chart makes sense to me.

5 --> 6 Very easy
10 --> 9 Easy
15 --> 12 Medium
20 --> 15 Hard
25 --> 18 Very Hard
30 --> 21 Nearly Impossible

For damage rolls, these can be the same, as what they oppose does not change.

For class features and spells that add random bonuses, It might make the most sense to convert them to static numbers:

d4 --> 2
d6 --> 3
d8 --> 4
d10 --> 5
d12 --> 6
d20 --> 10

As a nice reversal of this idea, you always roll two dice and add them together your modifier die and you d20. So if you are +3 you would roll d20+d6 and go against the DC. The only problem is there is not a nice solution to the missing d14, d16, and d18. Those dice types are made though they are very rare. You could do this, though in a pinch:
roll d8 reroll if you roll an 8 gives you a range of 1-7 then flip a coin or roll another die, tails/odd you add +0 to the 1-7 or heads/even you add +7 to the 1-7: this will always give you a range of 1-14 evenly distributed. Extrapolate this to the d16 and the d18.

Armor realignment would be -2 AC to the heavy armors, -1 AC to the medium armors. This would be an excellent rules tweak to add in DR to armor. Since, if you have a high DEX you get +5 AC, what purpose does armor have for you if the highest heavy armor gives you +5 AC... well I guess if you have a lower DEX...