Whoops, I forgot this was a "one thing you'd add", and not a general discussion of THAC0 ;-P ... I mean of topics.
I would chime in with "weapon and armor distinctions" (breaking the wish rules by using "and"). As someone else said pages earlier, the PF2 weapon table makes every weapon slightly different; that's an easy place to start. I cannot stand that the only distinction between warhammer, longsword, and battleaxe (besides the picture) is the damage type... and after 3rd level, that hardly matters because everything after "skeleton" that resists damage resists all mundane types!
I want the fighter to look at the giant scorpion and think "should I use the axe, and try to chop off a leg or that stinger? Or should I use the spike on my warhammer to puncture that hard shell? The sword stays sheathed... it's of no use here!" I don't necessarily need the 1e "weapons vs. armor" table... no one memorizes all that, even for the own weapon (even when it was helpfully on the character sheet!) -- I'm looking for a minor flavorful/elegant distinction that can be generally applied, but makes, say, the rogue's d6 sword better than her d6 hand crossbow in certain situations.
[I do this now with homebrew creatures and homebrew weapons. The Battlemaster specifically used his warhammer against the bludgeon-vulnerable kruthik-things, where he usually prefers his serrated (reroll 1s on damage) longsword...]