Zerg? They are kinda a blend of Species 8472 and Borg. Maybe they come from an alternate timeline/future where our Captain failed to stop the Borg assimilating the Undine. Or some extra-galactic threat.
Robot enemies. Not Androids, robots. A giant robot fleet is invading the Milky Way, in its ever-lasting quest to make the galaxy a safer place for their masters. Some kind of slug/snail/frog creature living on swamp worlds. They don't always resort to conquest and annihalation, but when they believe peaceful negotiation won't work, they do. Unfortunately, their First Contact was with the Borg. Or Kazon.