I don't know if it's been done before, but in this thread, you can make up your own roster as to which superheroes, cosmic or otherwise, you feel are worthy to be Guardians of the Galaxy.
I'll start with my own roster, which happens to be a large roster, consisting of the original 13 members from the 2008 DnA comics as well as other superheroes. If the Avengers and the X-Men can have large rosters, why not the Guardians? Think of it as Justice League Unlimited, GOTG style!
Star-Lord
Gamora
Rocket Raccoon
Drax the Destroyer
Groot
Moondragon
Quasar (Phyla-Vell)
Cosmo the Space-Dog
Major Victory
Adam Warlock
Jack Flag
Bug
Mantis
Angela
Deathbird
Lyja the Lazerfist
Nova (Richard Rider)
Thane
Beta Ray Bill