Kite474
Explorer
Eberron is cool but as I said last time, it strikes me as a low update priority.
The Realms gets a new setting book each edition. So it's a must. Plus the land changed with the Sunder, so people need to know what's what and where's where.
After that, things get fuzzier.
Eberron and Dark Sun were both upated in 3e, so that makes them less of a priority. Adding to that, Eberron hasn't changed since 3e, with no timeline advancement or alterations. And since the world is fairly new and books well printed, you can pretty much buy the 3e and 4e books for fairly cheap. You could get both 4e Eberron books for the same cost of buying a theoretical new campaign guide. Or 2-3 books for 3rd Edition. Or probably 5-6 in PDF format.
As the old information is still valid, there's less incentive to buy a new book since it has less to offer. It's a reiteration of what is known.
After that is Ravenloft and Dragonlance. Popular settings, but updated as licensed worlds in 3e. Slightly more likely, but still available.
Higher priority might be something that hasn't been updated in a while that could benefit from a full colour professional product, having not seen a hardcover product since 2e. Like Greyhawk. Or Mystara. Plus, WotC might need to begin worrying about their trademarks for their older settings.
Planescape would be a good choice since that could be folded into a Manual of the Planes product.
Also a good idea would be Kara Tur and Al Qadim, since those are different but technically expansions of the Realms.
This assumes though that they are updating by a priority of older settings though. Generally what it looks like is they are trying for settings that have a special "something" that is easily sell-able. It also assumes they are releasing setting books which has a snowballs chance in hell.
For example:
Dark Sun: Sword and Sorcery Dessert Adventures with an Eco-Friendly bent
Eberron: High Magic as high as the Empire State and Pulpy High-flying adventures
Ravenloft: Gothic Horror in its entirety
Planescape: Wacky Philosophical Planar Adventures! + It had a critically acclaimed video game whos story is still considered top tier writing for the industry.
Dragonlance: Not too sure about this one. A wee bit unfamiliar with it.
As for the other two:
Greyhawk: At this point everything interesting about it has been plundered and used everywhere else. It also does not intrinsically bring anything new to the table
Mystara: Has the problem of being a ho hum "generic" fantasy setting that has to compete with the Realms for appeal. It did have Shadow Over Mystara though so maybe it has some life in it.
All in all I think what determines which settings get brought back from sleep will depend entirely on marketability.