Call it the "power gamer's trait" and give it insane combat boosting components and facilities that act like the Value Improvement Plant, only for a whole SYSTEM... oh, wait, we
have one of those already
Seriously though, a few I'd like to see:
Antimatter Based (dunno what that would do in terms of game play...)
Energy Based (kinda like Organic only using lots of radioactives and energy)
Natural Spacefarers (aliens who live in space not on planets)
Heatmisers (can't think of a better name, basically life-forms who live inside of stars instead of on planets, like the Sonne are supposed to)
And basically any other sort of "extremophile" - aliens who live on neutron stars, in close orbit of black holes, in nebulas, etc. (Hey, why can't gas giant races colonize nebulas? That would be interesting...)