I'm not sure if this is a cop out, but I think that if there are 100 replies to this thread, you will get 100 different and possibly incompatible ideas.
In my opinion, that is what makes D&D and its 3PP diaspora great. It's not the best designed game, and never has been, but that has never stopped a large number of fans with substantially diverse TTRPG interests from playing it successfully and joyfully throughout the years, and deriving innumerable stories about some very different topics from it. None of us want the same things from it, and yet more of us stick around than not.
With the exception of a few niche generic systems, nearly every other RPG is concept- or setting-locked to a degree that requires some technical conversion to adapt to other playstyles. You can get surprisingly far with RAW D&D just by reskinning things.
If I knew how to recreate that in a vacuum, I wouldn't explain it here, I'd publish it.
I guess if I had one concrete request, it would be to reintroduce a bell curve to the core mechanic.
In early D&D, the bell curve was baked into ability score generation -- you rolled 3d6, in order, and if you got an 18 in Strength, that was a 1 in 216 chance and you'd already beaten the odds, so you picked Fighter and every Strength check you made during the game with a simple, flat 1d20 was 90% successful on average, and that was okay.
Because your character got crushed by a falling slab at 3rd level, and you rolled your next character and got a 3 Dexterity and smug condolences from your fellow players. The wheel of fortune turns!
Ever since the move to linear ability score modifiers, the game has gotten very swingy. Can't argue with the simplicity, but the die has a linear 20-point spread
; in D&D5, your character sheet doesn't become more relevant to success than that doom until about, what, 8th level, with some focused development? Probably closer to 10th or 11th, right at the end of the sweet spot.