The story of StarCraft still has some potential for another sequel. There are still plenty of loose ends to work with.
1. The Tal'Darim. After declining to join the reunited Protoss, will they declare war?
2. Brood mother Niadra. What became of her and her isolated group of Zerg?
3. Regrowth of life on barren worlds. Kerrigan and Raynor gone Xel'Naga?
4. The United Earth Directorate and their actions away from the Dominion, Protoss, and Zerg. Maybe a civil war?
5. The Confederacy. Supposedly eradicated, but things in StarCraft have a tendency to return from the dead. Just look at Stukov.
6. Speaking of Stukov, what did he do when Zagara took over? Is he still an unusual servant of the Zerg?
7. With Protoss culture changing dramatically, could we see a Terran become part of the Templar?