Who will we fight against in Wings of Liberty?

There's obviously going to be at least one new race for singleplayer only (even if it doesn't have a full techtree of units), such as the Draenai and Naga in WC3 TFT. But nothing indicates we'll be able to play Xel'Naga specifically, that's just wishful thinking as you said.

As far as mutliplayer goes, I doubt it. Maybe for SC3.