Hmm.... maybe there will be a galaxy-wide invasion by the Xel'Naga, the highly advanced alien civillisation who created the Zerg and the Protoss. The Xel'Naga had been in hiding for eons and were spending the time watching over the galaxy and preparing for galactic conquest.
The Terran, Protoss and Zerg forces are quickly overwhelmed. They suddenly realise that they have to overcome their differences and work together if they want to stand a chance of survival.
However, even the strength of the newfound alliance is still no match for the far superior Xel'Naga forces. The combined alliance is hunted down and destroyed almost down to extinction.
One day, while fending off a Xel'Naga attack on their homeworld, Jim Raynor and the Dark Templar Zeratul accidentally discover a lost Protoss Temple of the Hoo-ah which fortells the rising of a 'chosen one of the darkness' who would rise to bring light in the darkest hour. This chosen one would someday slay the 'source of the darkness'.
Following the clues in the Temple, they realise that the chosen one is none other than.... Kerrigan, queen of the blades! They also realise that she would be the one to face off with the head of the Xel'Naga in a death duel. They must somehow capture her and reverse the Zerg genome that she has in her in order for her to successfully challenge the Xel'Naga emperor.
Of course, the Xel'Naga realise this and decide to do everything they can to stop Raynor and kill Kerrigan before the prophecy is fulfilled.
Stupid plot huh?
I suppose the execs at Blizzard Entertainment will be vomitting blood all over their laptops if they were reading this thread.