I was thinking about the same thing also you have to remember theres a few troll islands and the goblin kingdom of undermine floating out there.

There's been speculation of the next expac being the Maelstrom or the Emerald Dream, is there any reason it can't be both at the same time?

the shared expac thing was a thought i'd had for a long time
Here's my rundown of the apples, the oranges, and both.
Th Emerald dream has the potential for content, but not enough content. Unlike Northrend and Outland, there isn't enough recognizability among the story-oriented playerbase. We know the Green Dragonflight likes to chill there, and we know some bad stuff is going down. That's about it.
The Maelstrom is far and away a more likely candidate between the two, as it allows for integration of unused Azerothian content (lore content and actual unused zone content) to couple with the new Maelstrom content.
As for putting them both together, here's the roadblock. They have nothing to do with each other. Beyond the looming threats both represent for Azeroth, they just don't share a playbill. You'd have more luck getting Phil Collins and Peter Gabriel to share a stage than these two ideas share a box cover.
Just my 2 cents for extremely large values of 2.

@ Craig R
What do you mean they have little to do with eachother, the maelstrom wouldn't exist in the emerald dream and would just be land, because the sundering would never have happened. matbe the devs are sending us through maelstrom to the unseen parts of azeroth. from an rp stand point it would make sense, because only druids and green dragons can enter the emerald dream, so one of those groups harnesses the power of the maelstrom to send the players through. :P

@Craig R the maelstrom and emerald ream are in fact almost DIRECTLY tied to each other. The first portal the demons were making was destroyed by Malfurion Stormrage (who i really hope will make his return if the rumors of maelstrom/emerald dream turn out to be correct) via the emerald dream.

It also makes sense that the emerald dream would be utilized against an enemy where a lot of land doesn't exist. The emerald dream shows what the world would look like if there had been no life, and there are multiple layers to the emerald dream. I could speculate for a long time on this, but in short, the emerald dream and maelstrom are very much connected

I think perhaps Adam has missed the obvious answer to this question. The name "Into the Maelstrom" is simply one that's been bandied about. It's not as if Blizzard has announced such a name or that play would even be entirely focused around such.

Even so, such a name would likely describe the expansions end-game content. Clearly, a "Maelstrom" expansion would be focused around the numerous isles and perhaps even continental areas of the South Seas. We know there are troll kingdoms there, though I believe anything major was destroyed in the Sundering. The goblin capital is in the south seas, and following their faction-neutral history, Undermine would like be the "Sanctuary" of the expansion. Pirate factions could also play a large and interesting role.

And coming to the Maelstrom itself. Queen Azshara is one of the very few remaining arch-enemies, corrupted by Sargeras, dwelling in the underwater Naga capital of Nazjatar. While there are hints in WoW that some people think Azshara is dead, Blizzard would have absolutely no issues overcoming any lore to explain why Azshara is alive and planning her assault on the surface world (or why Blood Elves can be paladins, or why Draenei are annoyed at Arthas, etc).

The south seas/maelstrom have a massive amount of lore centered around them. Far more so than even the Emerald Dream. It is very likely the next expansion will be focused in this area, but don't worry - it won't mean you're swimming everywhere, with perhaps the exception of entrance to end-game content, in a manner similar to Serpentshrine Cavern.

SPOILER ALERT

Further note, as I just finished reading the War of the Ancients trilogy, the text implies that Azshara and her naga were saved by the powers of the Old Gods. Somewhere else (and you'll have to forgive me for forgetting the source) implies that Ysera's battle in the Dream is against the Old Gods as well. Such a theme might tie the two different regions together, though I'd question if they would make an expansion focusing too heavily on that element since it seems doubtful that we've seen the end of their influence in Northrend yet.

"You'd have more luck getting Phil Collins and Peter Gabriel to share a stage"

I wouldn't consider that lucky

I see a lot of people commenting about how the Emerald Dream wouldn't have enough content to be its own expansion, but I don't tend to agree. First of all, the Emerald Dream could be potentially massive. It would be Kalimdor without the influence of sapient races (thus, no sundering. It'd be one giant continent) so there's no trouble with it not being large enough land-wise. Also, we know there's something major going down... The nightmare is drifting about the dream, corrupting everything in its wake, and its boundaries are growing. There has been hints that it may be connected to an old god. It would make sense for the major plot to be helping Ysera and the druids try to repair the damage caused by the nightmare while investigating its cause and seeking a way to stop it. This could create a variety of zones, both based on the uncorrupted portions, and zones in various stages of corruption.

But, being a realm without sapient races, where would we get our quests? Well, the two obvious groups would be the green dragonflight, and the druids of the Cenarion Circle (perhaps even shamans, as they are also very spiritual and have a strong affinity with nature, though their realm of expertise is more with the elements). But there could be others! I would not be surprised to find more nature-oriented races, such as the dryads, the sons of cenarius, ancients (tree-like guys, such as those patrolling Darnassus), etc. Also, it's a sort of spirit realm, or realm of dreams. Perhaps you run into spirits of the dead (Cenarius was killed, but his spirit is with Malfurion in the Emerald Dream, so why couldn't other souls reside there?), or people who are asleep. (Maybe they're there only briefly, or perhaps they have become trapped thanks to the influence of the nightmare.)

And given that the Emerald Dream is a realm of dreams, perhaps you could go back and forth between physical Azeroth and the Emerald Dream to help one side or the other, since they are connected.

I'm sure Blizzard is easily capable of thinking up content to fill an Emerald Dream expansion. You must keep in mind that we don't TRULY know a great deal about the Emerald Dream, so there could be much more there than we are aware of. (And even if there is nothing at the moment, the fact that we are only sure of a few details means there's more than enough room for new lore, so long as it doesn't blatantly contradict what we already know.)

Don't forget Deathwing, it's been shown in the war of the ancients trilogy that he knows his way around the emerald dream too... which could also be how the old gos are in there i suppose. If indeed it is the old gods.

However i wouldn't be surprised if the emerald dream and the maelstrom will have something to do with each other.