So, what I would personally like?
I would like a few sourcebooks/campaign books with themed options. Specifically, Al-Qadim and Kara-Tur (Oriental Adventures).
On the one hand, I think this would be magical, amazing, and (especially for younger players) open up worlds of imagination and further exploration. I know that we've got some inklings of flavor in the published subclasses, but I want thematic wholes. I still remember how the publication of OA led me down an amazing and awesome rabbit hole of play and discovery.
On the other hand ... it would have to be approached with care because times have changed. Pretty sure you can't just call a book Oriental Adventures anymore, and for good reason. Still, it would be amazing.
I agree with this, and I’ll add that there are a lot of people making great products (from games to novels to comics) that draw on their own culture’s mythology and fiction by people of that culture, and I’d love to see someone like Saladin Ahmed brought in to help write an Al Qadim book, for instance. Dude knows his Djinn, and has already created some wonderful fantasy worlds with heavy inspiration from the Muslim Near East.
The other thing I want more of is more products that explore the depth and breadth of what dnd can be.
Modern dnd.
Space dnd. (Big dream would be wotc getting back the Star Wars RPG license)
DnD in a world rife with magic, where humans are a minority species.
more races that push boundaries, like pixies, and races that help fill out campaign themes, like Vryloka, Dryads and satyrs, thri-keen, nocturnal desert dwelling gnomes, fey ogres and goblins, weird stuff like Wilden, playable vampires and werewolves, etc.
a full summoner class, where summoning is at least as big a part of the class as Wild Shape is for Druids.
More rules and examples for tinkering, alchemy, and magic item crafting.
More Ships (incoming, it seems), taverns, businesses, organizations, followers, keeps and domains, with no hard level requirements. (Sometimes it’s fun as hell to let the party keep that ship or shop or castle they win from enemies, and make it both a group resource and a narrative device, from early levels.
There are still subclasses we dont have, like a more strongly magical Ranger, an Arcane/Eldritch Monk (not another elemental evocator), a ranged weapon Paladin, Dragon Warlock, Warlock Rod/Staff pact boon, Primal Spirits Warlock, heavily Fey Ranger Monk and Barbarians, a dagger master (thrown weapons expert) rogue, that acrobat rogue mike was working on, a fighter or Paladin subclass with a follower, some kind of proper witch that calls upon spirits and curses stuff, mixing stuff dnd puts in Fey options, Druids stuff, and arcane Warlock+wizard stuff, a rogue with exploits, a sohei Monk with some divine stuff and able to use polearms as monks weapons....
also feats. More race feats, class specialization feats, tool feats, more weapon feats (make crappy weapons good with feats!), and more weird RP focused feats.
More items! Or even a book that collects all published magic items with errata, with some new stuff, variants, advice on tweaking items and making new ones, advice on handling different magic item assumptions, and all that expanded material i mentioned about tinkering, alchemy, and other crafting.
More spells, especially for classes that have issues, like Ranger.