Well, one of the main NPC factions is going to be the Imperium Galactis AKA the Galaxian Empire
They'd be the main "human" faction with their homeworld being Earth, nowadays called Terra Sacra.
A long time ago their dominion spread all over the galaxy, using a network of Giant Galaxian Gates (G3) to quickly travel anywhere in the Milky Way.
But then series of alien rebellions and civil wars reduced the Empire to only a shadow of itself, with most of its superior technology forgotten.
They are still a major power, controlling much of the Sagittarius and Orion arms of the galaxy.
They are warlike with massive fleets and economy supporting them yet overall they lack flexibility.
The Empire acts slow but with almost unrivaled military might.