I wouldn't mind a Dark Sun game, and I might even buy an updated DS campaign setting. However, it's pretty low on my list... I'd rather see them throw the designer resources at other things first. Eberron is my campaign setting of choice, for example.
A big hardcover "Worlds of Adventure" sort of book with just enough to get started on DS, Ravenloft, Al Quadim, and Planescape I'd buy in a second. That might also be good marketing. I understand the randomized minis work well because people will buy a box even if they don't like each individual mini, so there is less guesswork in predicting product demand. Throwing a bunch of campaign settings together might work the same way. Fans of all four settings would all buy the same book, possibly making it financially viable as a whole when it might not be individually.