The difference between a Vang type race, and our current biologicals is that the Vang dont make the planet glow in the dark, they subsume the resident population into Vang.
So, the planet isn't uninhabitable, but its damn hard to get the Vang off the planet with out resort to Very Bad Things to remove them.
The mention of parasite races is a good point, something like Hal Clements "Needle"
Or a race that sends out automated "Seed ships" that pre-terraform worlds prior to colonist arrival. That might make for an interesting robotic enemy outside of our currently well know and loved psudo-Cylons.
It would be interesting to see some racial characteristics added that might make the game play a bit different. With the current trends of planetary discovery and the discovery of more and more extreamophile life, you could see some pretty interesting aliens.
High pressure life isnt one of the more radical ideas. Deep ocean life on Earth operates at incredibly high, and very cold temperatures. Something in a high pressure atmosphere wouldn't be out of line. Current research is showing that higher pressure extends the habitability zone of a planet out substantially past what is currently referred to as the "habitable zone". As the pressure goes up, heat retention and heat distribution go up, insuring water stays liquid, even as the energy delivered by the star drops.
So, low temperature, high pressure worlds actually work as habitable, since the pressure acts to keep water liquid. And thats just assuming good old oxygen based life.
That could lead to interesting conflicts. Humans want a hotter, lower pressure, and the aliens going for colder, higher pressure. And thats just conventional stuff, some of the theories suggest far weirder possibilities, although as the heat index rises, the likely hood of complex life starts to decline. Venus like words, are still out of the realm of possibility, as they are way too hot, and way too pressurized.
Still, some interesting possibilities.
