It sounds to me like Forgotten Realms is actually poised for something of a comeback, as the "spellplague" reset button allows the setting to shed some of its cumbersome continuity while setting everything up to play smoothly with the new rules. I haven't played FR since 2e, but I'd consider running it if their changes achieve the goal of making it accessible.
I suspect this has been discussed elsewhere at length, but it will be quite awkward if/when Eberron makes the jump. The entire design philosophy of Eberron was "take everything in D&D 3.5 and put it in a world where it all makes sense." Even tiny details, like gnomes' attack bonus against kobolds, were supported with history and geography. And would Eberron still feel special in an edition that has stolen some of its best tricks?
Dragonlance has always been a setting plagued by (at least a perception of) caricatured races and limited story options for characters who aren't the heroes of the Chronicles. My understanding is that a lot has been done to address those issues, but the more you make Dragonlance about new heroes the more it blurs into generic D&D in my mind.
Dark Sun and Ravenloft suffer from being relatively narrow as settings, but perhaps they could be done as self-contained campaign setting/adventure path releases. I don't know of anyone who's ever played more than one campaign in either world.
Spelljammer seemed absurd to me even when I was in middle-school, but the core concept is creative enough that a complete overhaul from the ground up could be something special. I would love to see a new version of that weird space opera fantasy setting as envisioned by current D&D artists like Wayne Reynolds, William O'Connor and Steve Prescott.
Planescape was too wonderful and too of its time to hope that its success can be reproduced, but I bet before 4e rolls into another edition, there will be some official try. In fact, core D&D's move away from the Great Wheel could be incorporated as a story element. Perhaps it could result in some sort of planar cataclysm that brings back the Lady of Pain, shuffles the factions a bit, and provides plenty of adventure hooks.
