Well, they're still supposed to be making Planescape, though I suppose that could be considered niche also; a lot will depend on the execution.
"Snakes on a Plane of Existence," if you will.
TBF they COULD make more realms (not my first second or 5th choice) or Ebberon content too. The thing is people hombrewed spelljammer and planescape and darksun if they were die hard years ago...
Today yeah you can put out something... but we have had 8 (and by the 50th 10) years... the system isn't just what THEY or 3pp did, it's every homegame too.
When we started our most resent game one of the quistions we asked at session 0 is "what is everyone playing" and it always has 1 or 2 of us gunhoe for something and the rest asking "what haven't I tried"
by the end of 3e/3.5 we all still had lists of things we had not tried (but we were sick of the system) by the end of 2e, and 4e though we had gotten down to "I have played this already OR I'm not interested in playing this"
We have started to hit that "I already did, or I don't want to" part with 5e. I am sure a bunch of people have.
1 new class (artificer) and a bunch of subclasses have been relased, but once you have played an arcane trickster and an assassin, and an investigator you most likely are ready to move on from rogue.