Hmm... But from the way Blizzard works, I can sense that they might surprise us with a Warcraft IV to continue the plot, then create another WoW Expansion after or just another genre of Warcraft Games (RTS, RPG, etc.).
Remember, almost all of Blizzard's games are awesome, so if they make Warcraft IV, there would be a way for them to promote their other games too, or even make a newer one.
PS: I stand corrected, all Blizzard games are awesome. (Including those not so well known ones)
PPS: Although to me, Warcraft III: The Frozen Throne is already an awesome game, so if Blizzard found a way to connect Warcraft III (It's editor and games) to Warcraft IV (With a even better editor, and 2 other editors with it, the World Editor and Galaxy), I'd be crazy about it.