Within the freshest Starcraft 2 panel from BlizzCon 2008 Blizzard proclaims, that Starcraft 2 will be expanded into a trilogy. Furthermore Blizzard verifies, that Warcraft 4 will be released with the same principle.
Blizzard Co-founder Frank Pearce on videogaming247.com
“If there’s a team that’s passionate about doing another WarCraft RTS, then that’s definitely something we would consider,”
“It’s nothing that we’re working on right now,”
“We have development teams working on Wrath of the Lich King, StarCraft II, Diablo III, and when those teams are all off the projects they’re working on, they’ll be intimately involved in the the discussions about what’s next.”