Hello,
I'd like to suggest some sort of marine life.
There are ocean worlds but no one "really" inhabits them. Also it strikes me as rather odd that plants and funguses are gallivanting around the galaxy ;-) while no marine life seems to have made it to the stars...
Even Master of Orion back in the 90's had the Trilarians, many forms of "crab people" have been concieved for many movies and series (and books), – and on "real" earth we have lungfish, already highly intelligent octopodes, lots of creepy deep-sea-life (the anglerfish for instance), funny jellyfish, dolphins (although they're like the fairies of marine life), seahorses and so on. So in short: lots of fun animals to base spacefaring species on.
Some probably could be able to walk and live on land (-> Lungfish), some might come with (water-filled) space suits (similar to this one bird race with space helmets), some might just have water-filled tubes/hoses leading to their gills etc. etc.
(And something looking like the "crystal ball"-robots with the ball being an aquarium for the species might be fun, too)
And now I'm done ;-)
Cheers!