My Guesses:
Okay, as I mentioned above, I'm wildly guessing these are smaller PDF products like the Plane Shift PDFs for the Magic the Gathering settings. An adventure plus a brief setting overview with a few races and maybe a couple monsters. Perhaps a subclass.
This is an untested release method. They'd want to try it out with a few softballs before going all in. Classics. But maybe one weirder variant to give people *something* different that isn't generic fantasy.
I'm guessing Greyhawk or Mystara for one. A good old school option. Likely the former. It can be well served with a brief folio product, as was demonstrated in the early '80s.
The continued success of the novels make Dragonlance an appealing choice as well.
But Eberron is also a good choice and would pair well with the artificer. So I think that's a lock for the second option.
The limitations rules out something like Dark Sun that has a bajillion setting monsters, and likely too many new mechanics to easily update.
As such, I think the weird option is Planescape. It's safer than Spelljammer and is a subsetting that makes it easy to add to existing campaigns. And it's easily covered by focusing on Sigil.
But, these are wild guesses and WotC could surprise us...