I don't want it to be the same Starcraft story.
At least the origin.
Humans roam the universe for different reasons.
The dictatorship could be good if the leader does not seek absolute power, just as the defenders of the oppressed could simply be anarchists.
Let's add this as an ingredient of the human being out of curiosity, and to know secrets, as well as unconscionable and unethical scientific ambition.
All this gives a cocktail of different alliances and enmities between them, as well as conspiracies and a couple of human faction that deviated from the route in their journey through the universe.
Humans create (zerg) with his unnethical experiments, similar to Virus and biological weapons in resident evil.
Humans are trying to reach center of universe but they need reach the other side opposite to old solar system where the earth was devastated.