Making all races be Tasha's-style (no cultural characteristics in the races).
Turning all Planetouched into Lineages, a la the Gothic Lineages from Van Richten's Guide to Ravenloft (Tieflings, Genasi, Aasimar, etc).
Fixing the broken subclasses (4 Elements Monks, for example).
Expand upon Backgrounds mechanically and thematically (probably to fit the cultural features and proficiencies in them).
Allow for innate spells (either from race/lineage or feats) to be cast using your own spell slots.
Balance the feats. No more "you chose Linguist instead of Polearm Master?!?!" problems. (Preferably give more feats to the players, too.)
Make Druids choose a stat block that's included in their class features to Wild Shape into, instead of having to search through the Monster Manual and similar sources.
Give the Tasha's optional class features to the classes (and probably give more class features that give meaningful player choices when creating their character, like how Warlocks have Pact Boons and Eldritch Invocations).
Include Artificers in the PHB, and give it as many subclasses as the rest of the classes get (no more Wizards and Clerics with 7-8 subclasses when Sorcerers, Bards, Barbarians, Druids, and Rangers only get 2 subclasses!).
Give each class that has spellcasting more unique spells (imagine an AoE-version of Chaos Bolt for Sorcerers, more Songs for Bards, inventing spells for Artificers, etc).
Make tools actually matter in the PHB.
Include stat blocks for Vehicles. (Maybe even a section for Spelljamming ships in the DMG, near the Laser Pistols and Antimatter Rifles!)
Revise magic items to actually be interesting and factored into the math of the game.
More interesting weapon and armor choices, instead of just "oh, I just got enough gold to get the superior-in-every-way Light/Medium/Heavy Armor! I guess I'll buy (and use) that!"
A Psion class!
A Martial, Arcane Half-Caster (like an Arcane version of a Paladin or Ranger)!
Fixing Action Economy!
Mythic Monsters in the Monster Manual. (Mythic Tarrasque, anyone?)
I could go on and on and on.