I realized the mechanic of "alien invaders" in my game is actually "disorganized space barbarian hordes that invade the civilized player's empire" :) So I want to pursue the barbarians idea.

Do you have some story idea/rationalization why there are "space barbarians"? And who they are, why they did not formed some sort of space empire, etc?

It doesn't seem likely that 'barbarians' would have the technological infrastructure to build space ships. So I think either they are buying spaceships from another culture or they have organic spaceships that they grow or breed instead of building. If they are buying the ships, they are probably working in crews, like vikings or pirates, with the leader being the one who got the funding for the ship. If they are growing plant or animal-like ships then big ones might work in crews, but smaller ones would lend themselves to something more like a motorcycle gang or that kind of semi-independent group. A semi-military organization like a ninja clan or other warrior tribe is also a possibility.

As far as where they come from, they might be colony worlds. Perhaps the mother country world to deliberately create colonies where the population was less educated, had less technology, and was basically a slave or serf to the mother world. Then, some might be abandoned as not profitable or having semi-failed terraforming, or too contested by a different mother world. Or perhaps a mother world had some disaster or civil war and left all its colonies to fall to a low-tech state except for what they can get from traders; naturally the first thing they would want would be their own ships, to get some trade goods (legally or otherwise) to sell to the other people of their own colony. Another possibility is that a planet may have been basically destroyed in a war, leaving a lot of refugees looking for a way to earn a living, or a faction may have gotten kicked off their home world, leaving them missing the pleasures of civilized life but unable to produce them, so that leaves stealing them or winning loot that they can trade for them. And the traditional set-up of a homeland with a limited amount of land, probably inherited by a firstborn child, would result in a lot of 2nd, 3rd, etc. children wanting to go out and capture enough wealth that they could buy land and start a family of their own. This seems like it would be especially common for a carnivorous species, as they would need enough land per person to raise food animals. But fructivores would also need a lot of room for an orchard or vineyard.

You could base them on Genghis Khan or the Golden Horde. A barbarian empire that was high-tech (for it's day) that spread out of their own land and conquered half of Asia/Europe.

You can have it be a Hannibalian group- as in, a multiracial(cultural) force that is lead through the efforts of some honcho capable of bringing them together. That allows you to have diversity in the enemy forces.... But yeah, they can have highly diverse technologies and tactics derived from each of their host planets, and recombinations and mixtures of enemy troops lead to particular requirements in dealing with them. I dunno, what others said are pretty good.

I realized the mechanic of "alien invaders" in my game is actually "disorganized space barbarian hordes that invade the civilized player's empire" So I want to pursue the barbarians idea.

Do you have some story idea/rationalization why there are "space barbarians"? And who they are, why they did not formed some sort of space empire, etc?

I realize this reply may be a bit late, but perhaps it will help others as well...

There are already a couple good ideas for rationalizing why there are "space barbarians" but here's a few more...

Piracy - a quasi-legal group of mercenaries that seek out the enemies of large governments and often step outside the law for personal gain.

Large groups that are the last remnants of an old empire that has fallen apart, blew itself up, had their sun go nova, etc, etc.

Large space travelling itinerant groups that like to move around a lot and doesn't respect any laws except their own.

Escaped prisoners, war criminals or prisoners of war belonging to a destroyed world.

Religious zealots - they don't need a reason for doing crazy things.

A mysterious species of Space Barbarians. No one knows why they're all dressed like He-Man, or why they ride giant space dragons (their ships), how they can be so dangerous when they use swords and axes, or where they even go next as their attacks seem so random and without any larger strategy. Rumors say their appearance hides their incredible intelligence and weapon technology, others say their dragons give them magic powers that they channel through their swords, yet others say their speed is so incredible that they easily evade your blasters/lasers to get within reach of attack; but in one thing they all agree: that no one sees a Space Barbarian fight and lives to tell the tale (then where did the rumors come from, I wonder?).

Perhaps the same technology that got them into space had some bad affects. Maybe their intelligence is suited to building spaceships and piloting but not many other aspects of "civilization." I read a sci-fi series once about bio-engineered aliens that overran their creators. What affects would growing up in a weightless environment have? Perhaps a complete lack of survival-of-the-fittest led to a drop in intelligence over time. Maybe their spaceships are run by a slave race? Maybe they undergo metamorphoses in life that leave most of them barbarous. Perhaps they are impanted with intelligence dulling chips by something else. Maybe it's a result of being able to interface more closely with their ship systems.

Lots of options.

You could base them on Genghis Khan or the Golden Horde. A barbarian empire that was high-tech (for it's day) that spread out of their own land and conquered half of Asia/Europe.

Or the Vikings. Simple farmers at home and expert boatbuilders, but with dreams of riches far beyond the horizon and violence the primary way to get it.

You could base them on Genghis Khan or the Golden Horde. A barbarian empire that was high-tech (for it's day) that spread out of their own land and conquered half of Asia/Europe.

Or the Vikings. Simple farmers at home and expert boatbuilders, but with dreams of riches far beyond the horizon and violence the primary way to get it.

Hmmm, Vikings... Maybe they could use mechanic like that: They have core world and occupied worlds. They live on core worlds and conquer nearby systems and use as occupied worlds. Now the twist, every 50 turns they abandon all occupied worlds and go back to their core worlds then there is 25 turns of peace and they start conquering again.

(lore rationalization: they have change of leadership or some other internal ritual/custom also they raid these worlds for goodies/tribute, not to live there)

Why did I instantly think about Conan the barbarian jumping around with loincloth and a space helmet.