Racial traits to be replaced with an optional list of racial feats, as in Pathfinder 2. I don't want gnomes only to be rogues, bards or illusionist spellcasters.
Two pillars of leveling up. One would be the classis (more hit points and bonus for attacks and save) and the other would be like unlocking slots of talent trees.
The return of monster/templates/racial classes, and the racial parangon classes.
A optional module about a different list of abilities scores, for example adding acuity (astuteness + perception), spirit (courage + karma/fate/luck/divine grace) and agility (fast reaction and coordination, des would be about pre-learn actions and precision and crafting, playing music, dance, maneuvers of martial arts).
Martial maneuvers. In the past I insisted a lot of times about samurai, shinobi and sohei as martial adept classes. WotC could hire Dreamscarred Press staff.
A system for sanity/madness as the one from Unknown Armies, but I suggest the pillars: violence, helpleness, reality, regret and loliness.
Allegiance and aligment together. Spells and other powers with key aligment can hurt enemies with same aligment but different allegiance (religion, race, family, brotherhood, tribe, country) and opposite aligment+allegiance (for example a chaotic sheriff with allegiance law who breaks rules to defend the order, or a zealot/revolutionary with good allegiance but bad aligment because he forgot the value of the mercy).