I think with the way Brood Wars ends, you can tell which general direction a Starcraft sequel may be headed. But most compelling was the bonus mission you get to play when Zeratul destroys the Zerg/Protoss hybrid only for Duran to reveal he is not a mere agent of Kerrigan's and that he has planted the seed of this hybrid on dozens of other worlds.
By this point, Duran sounds not just infested, but like something else entirely. He says Duran has only been one of his aliases and that he has been alive for several millenia. So who is he? A Xel'Naga? Perhaps a renegade Xel'Naga? Or maybe an ancient enemy of the Xel'Naga that has come to bring a purity of essence and form together in this new hybrid to pave the way for an invasion.
What remains most interesting to me is how the Terrans will play out in all this. Since it seems (and I could be grossly wrong about this) that Terrans were not founded by the Xel'Naga, they could be the type of wild card that none of the races had considered. And while they may not be as physiologically or telepathically advanced as the Zerg or Protoss, their potential may also be far greater at the same time.
At any rate, the possibilities for Starcraft 2 are wide open.