Sounds like fun, as long as they can't spread to other planets (without hitching a ride) and can't burrow into your ship. They could, however, try and get into your ship and hide in corners, so you can take them to other planets to infest.
While exactly how it could be implemented effectively is a difficult matter (as it's a reasonably complex action to perform), I've always liked the idea of things that could get aboard your ship if you leave the doors open and, if there's a place for them to go that is hidden away, hide and multiply. Not something particularly hard to kill, per se, but something that's at least a pest and minor danger in groups or large numbers. Of course, none of these things would be hard-coded one-type Fauna. Theoretically, they would be subject to the same randomization/procedural variation as everything else, just be the ones that demonstrate these kinds of infestation behavior. (Among other behaviors they might display.) If you got unlucky, you might actually get something pretty deadly....
Keep tossing the ideas back and forth, everyone. I love watching all the brainstorming and fascinating ideas, whether they could be implemented or not. (Some things could be implemented, but the more intricate features would need to be cut out due to not being feasible or scalable. Still, the creative process often involves aiming high and then bringing the ideas back down to something doable.) Even if I don't post for a little while, I'm still actively watching this thread.
While infestation is a fun topic, it's a complex concept. At first, things will need to be much simpler. I'm curious: what kinds of Fauna would you like to see in regards to different environmental conditions? (An ice planet, a desert planet, a planet teeming with life, an arid planet, a humid, wet planet (I don't mean a water planet), a planet with almost no (or absolutely no) atmosphere....) Fauna that have adapted to either the kinds of planets already in StarMade or other kinds of environments that aren't too strange of ideas. (Don't take the list of planet types above as plans for StarMade; they are just typical environmental/biome concepts.)
What we do know is a good place to start for creating what we don't know. Perhaps think about some of your favorite animals and what about them you like, then start imagining different, unusual, exaggerated, fun ways of twisting those concepts and attributes. Or pick two of your favorite animals and mix things about them. Ignore the underlying principles of biology and just think of what makes them interesting.
How about frog feet/legs with massive claws? Arms that allow gliding like a flying squirrel, but in the shape of the arms of a praying mantis? The flexible bodies of octopuses, yet capable of moving nearly as nimbly on land as an octopus does in water? The thin membranes and perplexing anatomy of jellyfish, but floating in the air?* The ability to coil up into a wheel-like shell and roll across a planet, but uncoil into something vicious, nimble and aggressive? The trunk of an elephant with pincers and a lamprey's mouth at the end? (Yes, my job is to give you nightmares.) The locomotion of a snake, but with two small hooks near the end to allow it to hang from walls and off cliffs? (And even stranger, perhaps make that snake a gentle herbivore that just uses those hooks as a defense mechanism to hold itself where predators can't reach?)
Think about environments, climates and ecosystems. Think about hazards such as weather, temperature or how firm the soil/ground is. Think about the physical features, physical abilities, visual attributes or overall behaviors of your favorite animals, or any animal you can think of. And yes, fictional animals from Sci-Fi (or even fantasy) genres can be used for inspiration if they're not too crazy.
What ideas or mixtures seem coolest or most intriguing to you?
* : *cough* Metroids *cough* (Though jellyfish also have cool stinging tentacles.)