"STOP CALLING IT A NEW EDITION!"
-Wizards of the Coast, 2022
I'm happy with the current edition; it is pretty much everything I ever want from D&D. And for all of the minor things that I would like to see from older editions, I still have those older editions. (Why OSR, when you can just OS?) But in the spirit of this thread,
if none of this were the case, and
the next edition is called "sixth edition," and
if I still wasn't happy playing an older edition:
1. I'd want only four classes (Fighter, Mage, Priest, and Sneak). Subclasses, prestige classes, multiclassing, and all that jazz would be replaced with a modular system. Divide up all of the class features like Sneak Attack and Rage and Turn Undead, and put them into long chains of Perks that the player chooses from at every level, Diablo 2-style.
2. I'd also want to scrap spell slots, spell levels, and all that stuff, and use a new Mana system. It wouldn't be that difficult; spells have a level requirement (or a Perk requirement) and mana cost, done. Fireball unlocks if you have the Elemental Magic --> Fire Magic perks, and costs 9 or more MP. The more MP you spend, the more powerful the spell.
3. I'd want them to call it something else. As I'm sure everyone in this thread is about to inform me, this game would no longer "be" Dungeons & Dragons. And I'm okay with that. I don't need it to have that particular name, and in a lot of ways it's better if it doesn't--it might keep a lot of preconceived notions and expectations in check, at the very least.