Lets not confuse the "Conan the Barbarian" with what "Barbarians" actually were. Two most notable Barbaric groups from actualy history are:

Mongols, who were technologically superior and focused on 'conquest without government' - which is to say they primarily said 'You belong to us and we'll be back later to steal your crap.' - Which is just fine when you're basically defenseless against them.

Then we had the Germanic Barbarians, who fought the Romans. These were small groups of people who, due to changes in climate, were forced south to survive. They banded together sometimes to face the organized Romans, but for the most part were slightly lagging technologically and majorly lagging in their organization... but they were desperate and, due to the same climate change (Wheat Rust) the Romans eventually found themselves stretched too thin and the Barbarians were able to overcome their organization.

So here we have two very different barbaric ideas: The first is a technologically superior group focused on what basically amounts to large scale looting of "planets" (in this case), who use hit and run methods and simply come back to re-loot every now and then (also, re-rape, which is why so many people are related to Khans).

The other is a technologically inferior, disorganized mass being pushed by some greater force towards you, an organized and well defended planet. A relentless tide ever present and ever eroding your resources that would be put to better use if only they'd go away for a while.

Seems like both could fit a sci-fi setting pretty easily!

The other is a technologically inferior, disorganized mass being pushed by some greater force towards you, an organized and well defended planet. A relentless tide ever present and ever eroding your resources that would be put to better use if only they'd go away for a while.

A common sci-fi topic is the dangers of exponential population growth, either because of above-replacement birth rates or the existance of life extending technology. First one world fills, then another, then two more, then four more...

You could imagine such worlds continually throwing out desperate individuals searching for a home anywhere, building or stealing any ship they can. Perhaps a region of the galaxy has fallen into chaos and can't manage their populations and send desparate wave after wave towards your worlds. If you lose your grip on a planet, it becomes a source too. If you conquer the out of control planets and help them manage their population growth, the "barbarian" waves reduce in size until peace returns to the galaxy.

I'm probably more of a fan of the less-technologically advanced barbarians, since mongol-like ones are a little flatter in the story department. Barbarians in the Roman sense involved a negative outlook from the Romans. It was the Romans' opinion that being not Roman made these people uncivilized. Thus, maybe you could do a spin where the player plays as a some-time aggressor, though the "space invaders" certainly counter attack in return. Perhaps they're a tribal people that have very different cultural aims, but have been attacked by the player's group in the past, and feel threatened by you. Or maybe they launch raids to sustain themselves, since they will not join you, since that would prevent them from living according to their culture.