One man's musings - I think if you're looking at the older settings, you need to look at what makes them special and how would it be handled for 4th:
-Dragonlance - Epic storyline, lots of Dragons, high fantasy + magic weapons at the center of the major story...I see a lot of potential, but what was the favorite part of Dragonlance for most of the fans? The original storyline and that's pretty much it. So how would you handle it in 4th? Reboot. One book, "The world of Dragonlance" and one book "The Dragonlance Campaign" that would let players play through the original war of the lance. I could see this happening as both a return to a more typical fantasy world after Dark Sun this year and as part of a nostalgia movement gearing up at WOTC (Red Box Essentials, Tomb of Horrors, Gamma World, etc)
-Spelljammer - Very different, lots more planar-adventure opportunities and a possible coolness factor since it's been OOP for quite a while. It might be a reach as we're getting a "different" kind of campaign with DS this year. It would fit the World Book/Monster Book pattern very well. I think it would be cool but I'm not sure I'd see it as very likely.
-Ravenloft - Well vampires are trendy now, but not old-school creepy vamps. It's kind of a nostalgia thing, but more because of the single adventure rather than a campaign world. I think it would be tougher to make 4E characters fit into a domains of dread-style horror type game both mechanically and atmospherically. I don't know that I see this happening anytime soon. Plus this is partially covered with the boardgame, at least some of the nostalgia aspect is.
-Mystara - I played it but it's awfully similar to Greyhawk and the Realms and probably has less popularity/nostalgia than either. I can't see it making the grade right now as a separate campaign world.
- Birthright - Epic possibilities but it would need some kind of rules to run a domain and rules for mass combat. That would be cool but it's a pretty big change in focus from standard D&D. Without those things I think Birthright loses a lot of it's specialness.
- Greyhawk - This is the one I would rate as "likely" at this point. Nostalgia, fits with traditional D&D concepts(some of which 4E runs away from, admittedly) but when we have Giants last year, Tomb of Horros and a Red Box basic set this year, a Gamma World game coming out, MM3 includes a bunch of old-school monsters - all of that screams "Nostalgia" to me and Greyhawk fits right in there, more than any other to me. It seems to me that the 2010-2011 Gen Con year seems to be the year WOTC is making a push to reclaim the lapsed/disgruntled D&D player. After that I expect they will move forward, possibly with a whole new world.