In a word: Intentional.
What should D&D be? If we have tactical games (Draw Steel) story games (Daggerheart) and great games (Shadowdark) what is it that D&D wants to be?
I'd say it needs to try and be the jack of all trades, and a little bit of tactical, story, and straightforward, simple dungeoncrawl, all in one.
There are still things which D&D does, that it has gotten away from that could cement what 'D&D' is. All of this is IMO of course.
The Sacred Cows
This stuff needs to be there
Attributes (Str/Dex/Con/Int/Wis/Cha) - Not much to say here.
Ancestry (Human, Dwarf, Elf, Halfling, Tiefling, Aasimar, Half Elf, Half Orc, Gnome, Goliath, Warforged, Dragonborn, Goblin as a treat) - I think Daggerheart does Ancestry much better, with a much more elegant way of managing mixed ancestry, D&D could for sure lift from there, but if they do not...a solid list of options is a must.
Classes/Subclasses - The main class options are fine, but the design could be dramatically tightened up. I would shove multi-classing into a sidebar deep in the DMG, and would put subclasses at Level 1, and have something meaningful happen on every level up.
Alignment - This is a D&Dism, that absolutely should be leaned into. Put it into Class design, Ancestry design, Spell design. It should be fundamental to the game. If D&D wants to be this multiverse, planar, city of doors, spelljammer, planescape gonzo setting? Alignment, make it happen.
At the crunch level? Jesus clean things up. Masteries are fine in concept, but some of the theory craft is just horrific to read. Much like multi-classing, stick this deep in a sidebar as optional somewhere. Actually refine the concepts of the Wizard. It cannot, should not, be a dumping ground of just 'everything arcane'. Sorcerer seems to have a better definition that Wizard does now, its crazy.
Look at the design of games which are intentional in what they want to be, strip out the excess. Do not bother with rolled stats, just go with Array.
"Three Pillars" - If they are going to do it. Do it. Enough with the partial design, half measure stuff. Get the combat (done) exploration (not really done) and social (not really done) actually defined, documented, and put down in the rules.
And then, strip out everything that is not about the game, and toss it into a super clean SRD.
I dont know, I dont feel like it should really even be as difficult as it seems. The biggest thing missing, is the focus on Alignment to make it a truly epic "Good and Evil, Law and Chaos" crazy high concept Fantasy.
Do not try and be 'grounded' we have other games for that. Go wild, go planar, go multiple words and realms of the gods.