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.)