Tobold predicts that the next WoW expansion will hit in 2010, featuring another 10 levels, associated content, and a healing Hero class. I had decent luck with some very unambitious predictions for Blizzcon, so I say that the smart money is on slapping the title "The Emerald Nightmare" on that sucker and penciling it in for May 2010. But that's boring, so let's talk about a slightly more ambitious idea that isn't so much more work that it'd be beyond what Blizzard would be willing to consider: The Clash of the Dragonflights.
Why it won't be WoW: Murloc Maelstrom Menace
Before we get there, a few words on the only obvious alternative. One could very easily imagine an expansion set under the Maelstrom in which players fight the Naga and the Murlocs. Presumably the Goblin homeland, Undermine (on an island south of the Maelstrom) would be the capitol of this expansion, and there would be demand for playable Goblins (perhaps with the choice of which faction to join). I think this one will happen, but not this time. After two expansions with new continents, I think they'll want to do something slightly different. Also, there's nothing I saw in the Wrath beta to suggest that the Naga arc is moving forward this expansion. Going to the Maelstrom, which isn't really iconic in the previous games the way Illidan and Arthas were, right now would seem a bit out of the blue. (/pun)
The Emerald Dream
The Emerald Dream is kind of a parallel Azeroth, with a focus on nature. We've had ongoing plots from the original WoW suggesting that bad things are underfoot in the Dream, and those arcs continue in Wrath. Because it is a parallel world, Blizzard would essentially be taking existing terrain and making it greener and more overgrown - this is an appealing prospect given the length of time it's taken Blizzard to develop content. We also have a total of four portals that could lead into the Dream in Azeroth, currently home to the Green Dragon portals. Finally, the focus on druids would help to fit a healing-themed Hero class into the lore. Thus, this is viewed as the most natural place to stick the next expansion. (/pun)
Why don't I think Blizzard should stop with The Dream/Nightmare (which, again, they might)? Well, there shouldn't be much civilization in there, and I suspect that Blizzard will not want to make an entire expansion out of zones that are green. So where could they get more material to add some variety to the game?
Deathwing, the Undying?
Deathwing is the father of the Black Dragonflight, and he is supposedly still out there, messing with the Netherdrakes. Certainly not enough to base an entire expansion on, but he could set up shop in some currently un-utilised corner of the map (perhaps the dead World Tree in present day Hyjal could play a role in reviving him, more on this in a minute) and become a raid boss. (I'm sorry, I can't think of any good puns here. :()
The Bronze and the Infinite Dragonflights Fight for Time
Tobold suggested that what he would really want to see would be an expansion set in WoW's future that replaces most of the current content (effectively making WoW into its own sequel). I don't think that will happen as he suggests, but there IS a perfectly good mechanism for moving the timeline into the future via the Caverns of Time and the struggle between the Infinite Dragonflight (which is trying to mess with the timeline) and the Bronze flight (trying to preserve it). Perhaps all the loot that players have been pocketing while messing around in the past was supposed to play some role in the future, and now the players themselves will need to jump ahead to make sure that the leader of the Infinite Flight dies when he's supposed to.
Like the Emerald Dream idea, this would allow the zone designers to recycle terrain, and hopefully shorten the dev cycle. The expansion probably still won't be out for the Holidays in '09, but it could come out in time for early 2010. (The /puns are back!) That way, we'd be looking at 2 expansions in three years instead of two years per expansion.
Bringing it all together
So, what do all these dragon arcs have in common? Well, the Dragonflights are all united in trying to stop Malygos and the Blue flight at the moment, so perhaps their enemies will view now as the time to strike. The zone geography team could spend the time they save by recycling zones making old world Azeroth flyable (like it or hate it, it's going to make less and less sense for flying mounts to be able to follow you everywhere except the old world), and we would presumably see more of the dragon-riding mechanic that's in some of the dungeons of Wrath as players side with one of the good (or at least less evil) Flights.
Blizzard has previously hesitated on the flight issue due to the time, and the need to fill out some missing zones of the old world (notably present-day Hyjal), but there's no reason why Deathwing and/or the Infinite Flight can't take up residence in these areas. With maybe 1-2 new zones, it would be easy to get together the requisite 7-9 zones total - we would have Duskwood, Winterspring, Ashenvale, and Ferelas for the Green Dragon portals, perhaps a pre-Sundering version of Azshara via the CoT to set the stage for the Maelstrom expansion down the line, 1-2 future zones, and, just like that, they're already done. Players would be spread out in a bunch of disconnected zones instead of united on one new continent, but I'm sure Blizzard would love the opportunity to have players traveling through the increasingly deserted old world of Azeroth to get from zone to zone.
Realistically? Expect the Emerald Nightmare in mid-late 2010. Which is too bad, because The Clash of the Dragonflights would be fun. If I were sitting around bored in late 2009 - wondering should I stay or should I go (one last /pun for the road!) - a far-reaching, time-spanning war against the dragons would be a lot more to look forward to than an entire expansion done in overgrown green.