I honstly wouldn't mind something of the sort of Barbarians in Civ. A random 'race' that seems to have no home. (But may OCCASIONALLY have colonized worlds every now and then, or stations, as 'Pirate Ports') and occasionally attacks your ships and trading routes, builds fleets, retreats, tries to increase power solely by attacking and stealing things rather than the traditional government way.
That being said, it'd probably take alot of coding, and I'm sure Stardock has alot to do. Maybe for the next expansion?
Piracy on the high seas would be a good source for some behavior ideas, though, space is basically like a sea, same with a colonial rush from big empires to new areas, I could really see piracy taking off, privateers hired to atttack other nations, rogues, etc. IT sounds like it'd be awesome.
Man...now I'm jazzed...I want pirates in the next expansion (After Dark Avatar) Pirate bases, pirate ports, mercenaries, information brokers, and ship-jacking.

Ah...but I don't think I'll get it, as cool as it might be.