I'm thinking the third expansion of WoW will be the Emerald Dream, and it'll likely come out towards the end of 2010. There are already four gateways into it--one in Feralas, one in Duskwood, one in the Hinterlands, and one in Ashenvale. Currently they are guarded by dragon bosses and elites (do NOT go near them if you know what's good for you) and go nowhere, but one day, they will have to... No official word yet, but the Emerald Dream and the Maelstrom are considered to be two of the most likely.