When you really think about it... Making a Third installation of Diablo before the Second installation of Starcraft, would seem a little wrong. Now that we got SC2 coming, D3 is very close. Can't wait to play SC2!
It probably won't be the Xel'Naga. That would be like playing the Burning Legion in Warcraft 3. However, if there's a new race, I believe it may be a dichotomy race to the Xel'Naga. It is also my theory that this mystery race is the one creating the Protoss/Zerg hybrids.
3:26 - Showing another video of "things to come." concept art for SC2 - zerg landscapes - ruined terran structures - freen filds - SC2 looks, at least visually, strikingly similar to SC1 - both in unit and map designs.