Check out this post. I read it a long time ago back during vanilla, and a few times after that. It contains Blizzard plan regarding locations set per expansions as they saw it in 2003 (or earlier). So far everything turned out to be true...
Those guys in Blizzard never stop to amaze me... One of the very few companies out there that can have a 10 years business plan that works!
According to this info, we have 2 more expansions ahead of us after Cataclysm