--I would love AD&D ability scores. An 18 used to be amazing, and 20 was basically unheard of. Getting those Guantlets of Ogre Power and boosting your Str to 19 was something fighters looked forward to. Now, with a 19 being no different from 18, and most fighters having a 20 in Str by the time you'd get those gauntlets, they are rather useless unless you want to give it to a wizard or cleric so they can carry more.
Sadly, to get that to work within the framework of 5e, you'd have to change so much more. You'd need a "Spell To Hit Modifier" for Wisdom, Intelligence, and Charisma stats, damage bonus for Dexterity to allow finesse weapons to work (as well as bows), and quite a bit more that my brain doesn't quite want to remember this late at night. I had honestly attempted to implement such a thing, even going to adjust all the monsters to reflect the changes to ability score modifications, but doing all that seemed a bit too ambitious.
--Also, crit modifiers for weapons again. Greatsword used to crit on a 19-20, while a greataxe gave x3 damage. Now, the greatsword is the better of the weapons because you never deal 1 damage on a die roll.
Bring back a lot of stuff from 3rd edition (damage reduction types expanded for one), as well as a lot from 2e (the ability score thing, resurrection %, system shock %). Oh, and make something similar to Pathfinder 1e's animal companion thing. And divorce ASI and feats from each other as well as from class (similar to 3e).
