I want every class to have its own thing. If a paladin is just a mix between fighter and cleric, that’s multi-classing, not a separate class. It needs to do something neither fighter nor cleric can do. (Ditto any other"non-basic four" class; paladin’s just an example.)
I want things like mounts and familiars to be available from the get-go, and be meaningful choices. And I want funky options–like a paladin riding a ki-rin, or a wyvern, or a giant butterfly–to be feasible (perhaps with feats or special powers) if the DM wants to allow them.
I want weapon-and-shield to be a meaningful choice. There’s a reason it was so common historically. I don’t want it to be the "best" choice–no no-brainers–but right now, unless you’ve build your character heavily around the concept, it’s often inferior to heavy weapon or dual weapon.
I want different kinds of magic–arcane, divine, whatever–to feel different. Maybe one can do something the other can’t. Maybe one’s almost useless for offense, the other for support. I dunno. But I want there to be a marked and significant difference between what a wizard is doing vs. what a cleric is doing, and not just as relates to healing.
I want race to matter more than "+2 to this stat and darkvision." It can be minor–probably should be minor–but to at least a small extent, I want a 9th-level elf fighter to be mechanically distinct from a 9th-level dwarf fighter, and not just by a few numerical differences.
If the game includes a prestige class-like concept, I want them to be truly focused, and never no-brainers. There should be solid reason to stick with fighter or wizard all the way through max level.
I want the basic math to make no assumptions about quantity of magic items. Balance a monster’s level/CR/whatever assuming a party with zero magic items (or at least combat-related ones). Then give guidelines for low-magic, average-magic, and high-magic campaigns, including rules for "If you’re playing X kind of campaign, increase the average level of the party’s opposition by Y."
I want a return of the 2E guidelines for what equipment is available in different time periods/cultures.
I want there to be some classes that can be played as simply as "I roll to hit, what damage do I do?" without choosing powers or maneuvers. Not saying those classes can’t have such things as options, but there need to be some truly newbie-friendly options.
I want monsters to go the 1E/2E/4E route of being designed to purpose, rather than the 3E route of being built exactly like PCs.
I want the game to stop assuming X number of encounters in Y time period.
I want the drow to go back to being a mysterious, insular, vile, and non-PC-appropriate race, but I know that’s not gonna happen. 😉
And I’m sure I’m forgetting a bunch, but that’ll do for now.
Edit to Add: Yep, forgot one. I want warlocks to exist, I want them to have multiple "patron" possibilities, and I want said choices to have major impact on the character (and not just mechanical, either). If a warlock gets his power from a devil, that fact should come into play, and very differently than it would if he got his power from a lord of the fey.