Minimalist D&D XIV - Overlapping abilities (and Mental Defense)

There has always been some conceptual overlap between Intelligence and Wisdom. In the beginning, that was irrelevant. Intelligence was for magic-users, Wisdom for clerics, and other than that, they didn't do much anyway. 

Things have changed. In modern D&D, you will hardly make any check that does not include one ability score in some way. Saving throws, initiative, attacks, damage... all rely on your abilities to some extent. In old school D&D, class and level were more important. In 5e, it is about 50-50.

And the overlap has increased, too. Now you can cast spells with Intelligence, Wisdom and even Charisma. You can use your Strength to boost your AC with heavy armor, or you Dexterity with light armor. Some classes get to add Wisdom or Constitution to AC when not wearing armor (maybe Charisma too; I don't recall, but it would be fitting). Some weapons allow you choose between Strength and Dexterity for attack rolls AND damage. When escaping from a grapple, you can choose between Strength (athletics) and Dexterity (acrobatics), making both abilities AND skills overlap.

So many abilities become dubious and redundant. You can use a rapier with either Dex or Str, but if you have BOTH it doesn't really matter, because you're only using one. Likewise, if you're wearing heavy armor, your Dex doesn't matter -  which makes little sense.

This is not a huge problem in itself. It can lead, however, to discussions ("but I should be able to use my Wisdom here!") or dump stats (if I can choose what I want, I'll always choose the same ability).

To avoid that, I'd prefer to have some overlap - especially when the PC is "attacking" - but some limits - especially when the PC is "defending" (more about that here). I'd also like all abilities to be useful for all characters, so having both Dex and Str (or Wis and Int) would be useful. 

Here was how I'd do it.

Art by Rick Troula - source.

When you attack, you can use either Str or Dex (your choice). If the attack is ranged, however, you can only use Dex. And, regardless of the type of attack, you only add Str to damage. So, you need decent Str to shoot a longbow, as it should be.

Dexterity always affect your AC - but the amount of armor you can carry is determined by Str and maybe Con (it makes sense you need constitution to wear armor or carry heavy stuff for long periods). You might have noticed that AC is now an amalgam of Dex, Str and Con.

Magic uses a similar reasoning. It's up to you whether to use Int, Wis or Cha to cast spells. However, you do not choose how to defend from spells...

This part a bit trickier. The difference between a saving throw that uses Str, Dex or Con is more or less obvious. For Int, Wis and Cha, the separation is not so easy. Usually, Wisdom is the most common one - but conceptually, I could see a fighter or mage with low Wisdom and high mental fortitude, either because of courage (say, Charisma) or smarts (Intelligence).

I'm thinking that using a single "Mental Defense" trait would solve a lot of problems. An average of Int, Wis and Cha would be good enough. 13th Age already does something similar. I might call it "Mental Class" - an analogous to AC.

I dislike the idea of adding a trait to D&D; my minimalist version should be subtracting most of the time. But this addition would simplify the game, and mirror AC very well - being an amalgam of Int, Wis and Cha. It solves the problems of dump-stats - since now every ability is important, at least for defense - and avoids confusion about which saving throw to apply.