SC2 HOTS's story is centered around Kerrigan. She's a complex character. At the start, she's in a state of flux between her human past and her Zerg - influenced present. The game shows her transformation back to the powerful Queen of Blades. She wages war against Mengsk, who had wronged her in the past. The Zerg follow her lead as she makes strategic moves against the terrans. Along the way, there are interactions with the protoss as well. The story is about Kerrigan reclaiming her power and seeking vengeance while also determining the future of the Zerg race in the StarCraft II universe.
Perhaps one key element could be her powers. If she has unique abilities, that would be a major part of her story. For example, if she can control certain elements or has strong combat skills.