For me, as much as I genuinely enjoy 5E, this would be my wants:
One book. The core should be rules-light enough to cover all the essential player option, a good mix of monsters, treasure and whatnot. Rules Enclopedia did it once, it should be possible to do again.
Less focus on combat. The game slows to a crawl or leans to strings of combat. Round to round feels like swinging nerf bats. Each roll of the die should have a significant affect on the narrative and instead of being a blow-by-blow I’d prefer a combat roll to cover a significant sequence of events, and in the neighborhood of three rolls per PC to resolve (covering the opening moves and position, main combat and wrap up).
Finally, spells are too prevalent and specific. In many ways, they are autocomplete buttons for certain actions - replacements for skill checks ( which worked for old editions that lacked skill systems). I really like the way Savage Worlds handles spells - rather than having a half-dozen damage spells that vary by level, area and damage type, SW uses one spell, and you set the specifics when you use it ( or acquire it) with what is called “trappings”. Likewise, a spell like invisibility might be a generic spell that augments a skill check and could cover a wide range of spells - invisibility ( perception), darkness ( a “mass” sort of invisibility), silence ( affects hearing instead of sight), strength (affects strength skills instead of perception), enlarge/reduce ( affects size instead of stats), charm person (affects charisma skills) and so on.