So, I will say that folks expecting a TCOE/XGTE sized subclass supplement for Dark Sun are almost certainly going to be disappointed. Yes, past editions have included a large quantity of class content for Dark Sun... just like they did for every other setting, because the pre-5e TSR/WotC ethos was that they had to publish huge amounts of crunch to make books sell. That won't change with bringing back classic settings. Ravenloft's character creation chapter is a pretty good indicator of where we'll end up
- A few races. Thri-Kreen & Mul will be new, then a Goliath reprint as Half Giants. (Who knows, maybe Half Giants get unique racial traits like how Leonin aren't Tabaxi)
- Two subclasses - I can't see them ever going above 4.
- A number of backgrounds. A variant entertainer for the Minstrels makes a lot of sense, Veiled Alliance member, Gladiator, that kind of stuff.
- A boons system, in this case almost certainly Wild Talents.
- Some expanded general rules - Defiling for sure, maybe some water management details.
This is also why I made the OP - because I just don't see them squeezing in a full class in the Dark Sun book, and I'd much rather have a small collection of subclasses that plug other holes.
Eberron: Rising from the Last War.
Added the Artificer. Four new Races. And Dragonmarks.
Surely there's enough room in a Dark Sun Sourcebook to put in a Psion, one new race, two subraces, a quick description of race/class reflavoring, and a couple subclasses. And instead of the Group Patrons section they can just do the Defiling/Preserving rules.
There'd be plenty of room in 256 pages.
Heck. Look what I did with 7, and I used 2/5ths of most of the pages for art.
Dark Sun 5e Systems Resource Document - The Homebrewery
The KibblesTasty Psion takes up about 15 pages. The Artificer takes up 10. So there's a bit of a difference, there. But, y'know. 1-2 pages for Thri-Kreen, another 2 pages for Subraces. let's be generous and say 8 pages for Race Refluffing and contextualizing to go along with the 1-2 pages of Thri-Kreen. Add in another 10 pages, why not, to recontextualize the classes and add in 2 Subclasses (Elemental Cleric, Archivist Artificer).
That's 38 pages of character-centric stuff. E:RftLW has 61, including the Dragonmarks. While KibblesTasty's 15 already includes Psionic Powers, Disciplines, and Psionic Feats.
Group Patrons is 32 pages. In that we could put a whole "Journey System" with a few pages of Encounter Tables as well as the rules for Defiling and Preserving.
The Khorvaire Gazeteer and in-depth on Sharn is 77 pages. The Tablelands could easily fit inside that. That's 147 pages.
Leaves us with almost 101 pages for a Bestiary, overviews of different Groups, and a "Known History of Athas" that pretty much starts -after- Rajaat gets sealed away with hints to earlier events that don't go into detail and leave hanging threads for players to pull and DMs to grab from older Dark Sun editions.
It'd work just fine.