Considering that Olimar and co. have landed in the same part of the world in both games, he could always go to a different part of the planet, like more in the east or something. New planets shouldn't be introduced imo.
A return of the Forest Navel would be nice. If any stage returns, it should be that one.
Currently playing: Final Fantasy XIII, Final Fantasy Tactics, Final Fantasy IV: The Complete Collection, Radiata Stories, Persona 3: FES, and Pikmin 2.