Sufficiently Advanced has a lot of civilizations as it is, but there's always room for more cool stuff in an infinite universe.
We're going to be publishing a setting supplement at some point. If you detail a good enough civilization, well-written and carefully thought out, compelling and believable, it has a chance to go into that supplement. You'll be given full credit in the book, a free PDF copy, and a share of the profits (minimal though they will be - don't expect more than $50). The current plan is to have roughly seven civilizations in the supplement.
Post your idea in the forums! Encourage other folks to read and enjoy - and to comment! And if you see someone else posting ideas, be sure to comment on them as well - feedback helps us all become better writers.