The Greys would be a good idea, kind of like a human substitute, but we'd have to give them some kind of twist, and maby more of a rouge than spy race, perhaps remenats of a past civilization.
This could give rise to a spin off or subplot about the current R.P universe being an alternate dimesion thingy, were it's the universe billions of years in the future or something along those lines, i'm having trouble thinking this through so feel free to re-phrase it.
Just a few ideas concerning it, and i'd be willing to contribute.