I like this. I always thought Civilization could be cooler if the barbarians were actually occupying the land and you had to kind of take over their stuff rather than just serving as a sort of randomly-distributed thorn in your side.
Yeah, I was trying to figure out an "imagine if the barbs in Civ IV were..." way of explaining it, but failed.
The greatest resource in the DW galaxy is survivor worlds.
Empires want to 'colonize' them; which amounts to rather than landing a pittance of population on a habitable world and staggering through a long growth process you land the same pittance of population on a planet that already has enough taxable citizens to immediately be a contribution. This works really well if they are another fragment of the same collapsed empire that you came from, less well if they are a not all that different species, and not at all if they are too incompatible. Landing 30,000 humanoid colonists on a not quite space-faring population of a billion humanoids they are met as a great leap forward boon and put in control of society.
Landing them on some other similarly developed planets they are met as 30,000 handy meals and put into the food supply for the local population of sentient insects. In such situations landing a bunch of troops is more appropriate, but that's a more daunting project.
Pirate clans want to corrupt them. They build a fortress, rule, and exploit. Eventually if they aren't interrupted they will develop one into the center of an interplanetary criminal syndicate that has pretty much the same capabilities as any other empire.
If really left to their own devices long enough they will develop the techs that your fledgling empire had to start with and declare themselves their own empire. Their late start is obviously a handicap, but it isn't like any other empire generally gets off to a roaring start, so this adds more opponents and is mostly a pain in the ass.