Interesting. Sounds like that would be a much better candidate for this format than Planescape. I always considered it the poor man's Planescape, but then I am a snob.
Dark Sun is an interesting one to consider. I think the main problem is the "one and done" thing WotC does with settings. I mean, if you did a Dark Sun slipcase, you'd need to reprint all the psionic subclasses and probably a couple more, and the wild talents, and maybe some "weapon materials" and "alternate armour" and "desert survival" rules, and probably also defiling/preserving, and a new race or three (including a reprint of Thri-Kreen) and unfortunately that's probably like 40+ pages right there without even a Psion full class. It's easy to see how you could get a 64-page adventure (let's kill Kalak again - I honestly never get bored of killing Kalak!) and 64 pages of monsters for Athas, but trying to detail the setting in 20-odd pages? That would be challenging as hell. Otherwise this would be a great format for that.
Really, I think they need to look at upping the main book to 128 or more pages, that would solve so many potential problems.
Also if they weren't doing "one and done", they could have it be like "Dark Sun: The Fall of Kalak" or something and detail the other city-states later and maybe just manage to jam that all into that format.