What I have noticed...
1) Adventures. I don't know if I'm in a "supergroup," but trying to find an adventure that someone in our group hasn't already played is a real challenge. One player has done Princes of the Apocalypse, another Curse of Strahd, a third has done Out of the Abyss, etc. Having more than 2 or so adventures published a year would help alleviate this as well as giving the DM actual choices. (For example, if I don't want to run something set in hell because of someone's religious perspectives.)
Well, there are some DM's Guild adventures that exist, and you can grab those on the cheap. You could grab some old adventures and port those if you don't mid doing a little extra work. You could also approach the adventures already played in a different way or modify them, like running them as an evil campaign if you want to give that a try.
2) High level content. Most adventures stop at 11-13th level, and the bulk of content seems to focus on 1-5th level. I know you'll say that surveys indicate most people only play to 11th level, but having more content would help. Not only adventures, but also sourcebooks, crunch, and DM tips for running high level play.
I kinda agree with you there, but there's a thing where it's honestly pretty hard to both balance high level stuff and the fact that the adventure makers won't really know your party, and at that point, you have to know your party in order to challenge them.
If you want some high CR monsters, I know Mordenkienen's has a good bit of them with some neat gimmicks, plus you could always get 3rd party monster books (recommend Kobold Press's offers, they'e good)
3) Psionics. An actual real print book about psionics - and maybe other alternate magic systems to pad the book?
Psionics are coming, just very slowly, because adding a whole new magic system takes
a slight bit of work, and they don't just want to crab it out I imagine
4) The combat and tactics book. The transition from 4e to 5e. The crunch to expand tactical play options. I'm regularly asked by my groups to beef up the tactical play in 5e. I'm nervous to do so as the game might shatter. Also something to add extra personalization to the monsters would be great. (Adding auras, legendary actions, "controller" abilities, etc.)
I can see that, but it's honestly fairly easy to do that now, just a bit into monster design yourself. If you want some ideas, borrowing bits from other monsters (like giving a Red Dragon the Balor's fire aura thing) or giving monsters some class levels (like a Pit Fiend with Conquest Paladin levels) though I would be sparing with that.
5) Mass combat. (I know Matt Colville is addressing this in an upcoming Kickstarter, but WotC should have been leading the pack on this.)
Yeah, that's one thing that needs fixing. I know some UA came out a while back, but that was a while back.