Let me see here, i got a cool story...
At the beginning of the game, which will be a zerg campaign, Kerrigan catches word that Raynor took control of all Terran forces. She also finds out that Zeratul did the same with the Protoss. Also she hears that they have banded together to fight her forces and those of the Hybrid. Then she hears that there is a new force approaching the galaxy, the Xel'Naga. So the xel'naga help rebuild the wounded Protoss Nation, while attacking the Zerg, then the commander, the one i posted on CREATE YOUR OWN RACE/UNIT for sc2, teams up with Raynors main force because there is an oncoming army of 2 million hybrid forces, 1 million infested zealots, and 500,000 infested archons and 500,000 infested templar (dark and high). so they go to the main base of the terran force, they hear much noise in the grounds about 100 miles south, they go very high in a sience vessel and see the army unloading from their ships, then Raynor's like HOLY SHIT, and the xel'naga commander stands there, he looks down at his army which consists of only 750,000 High Zealots, Raynor looks at his 2,000,000 marines, but remembers that they haven't much armor and will barely stand a chance against these. all of the stuff having to do with xel'naga and terran are part of the Xel'Naga campaign. So then you are still in the cinematic, and the army is only like a mile away, so the gateways are bringing in high zealots and holy templars like crazy. Then the Xel'Naga commander sends his zealots to charge into battle, the marines go behind as ranged support, you see blood and dying zealots everywhere! then it puts you in, you are expected to destroy the forces on your own, u have terran and xel'naga forces at hand.... the rest is up to you!