I am a bit lost imagining which traditional dnd settings they are planning to revamp...
I don't think psionics are blocking Dark Sun... don't we already have psionics in Tasha's book? I rather think the problem is that there are no religions and very restricted magic in Dark Sun. You can't easily market a setting which makes the majority of the core books unusable.
What I think they could do is assign the domains to the various elements in the same way they assign them to gods. If you're a cleric of water, you can pick from the Life, Nature, or Tempest domains; if you're a cleric of Fire, you can pick Forge, Light, or War. They could even create expanded spell lists for each element and say that you can, should, or even have to replace the expanded domains spells in the existing domains with the elemental ones. E.g., it doesn't matter what domain you take. If you're an Air cleric, your 1st-level domain spells are
feather fall and
thunderwave (or whatever). If they include the paraelements, then they have even more options. Some of the existing domains, like Trickery or Death, might be hard to place, but they might represent rogue clerics (
not rogue/clerics!) who eschew the traditional elemental setup or follow a twisted version of a normal elemental or something like that.
They
could, of course, simply create four new elemental domains and say the other domains don't exist or are vanishingly rare.
Now, I'm not sure how they would do defiling/preserving magic, because I honestly can't remember how they did it in 2e.
Mystara and Greyhawk, I still don't know what distinct features they have. They are like a less developed version of Forgotten Realms, with less magic, less monsters, less factions, less stories, less everything.
I doubt that either of those settings has "less stories," since there are nearly an infinite number of stories that can be told in
any setting--including the real world, which has
no magic or monsters. All you need is creativity. I'm sure there are factions in Mystara and Greyhawk, and that those settings have enough room to add new ones without stepping on anyone's toes or annoying the die-hard fans. Provided that they're not just copies of the Realms' factions, of course.
What Greyhwak and possibly Mystara (don't know enough about that setting, beyond its kitchen-sink approach) would need is a bit of a primer on how to run gritty, lower-fantasy games. But they did small primers on different types of horror game in Ravenloft, so I imagine that it would be easy for them to spend a page or two on that. Most of the lower-magic aspects of those settings is that they were developed at a time when there stat and race requirements for classes so many of the caster classes were made rarer. It was harder to become a ranger or paladin (or illusionist) and bizarrely difficult to become a bard. The primer for a 5e game would simply need to emphasize that magic users of any type are a lot rarer than they are in the Realms.
Or, they could just revamp Forgotten Realms again. SCAG was regionally limited, they could cover a different area or zoom out over the whole Faerun. But I would expect this for after the 50th anniversary edition so that they can present another cosmic deity-slaughtering continent-rearrangin cataclism to represent that now everyone got a free feat.
As long as they stay away from the Sword Coast, which has been done to death. Bored Coast, actually.
