PedroSteckecilo said:
I'm hoping Lich King will be the last, mostly because I REALLY want Warcraft 4 to be released. I doubt it though, and things like The Emerald Dream could be fun.
Personally I think if they REALLY shook things up that would be cool, by like seriously upsetting the practiced playstyle for the next one. WoW has become to lamely loot grindy in the last while.
WC4 won't be done by the WoW team, it'll be done by the RTS team - currently hard at work on SC2.
If I had to guess, I'd say that WC4 and WoW could probably exist concurrently. And hell, it's not like they can't just invent new antagonists.

How many bad guys in WC3 came from WC2 again? The Ner'zhul part of the Lich King, and that's a stretch.
At Blizzcon, when someone asked Chris Metzen at a panel, he said to "not think continents," which says to me either... well, the Dream or Maelstrom, pretty much.