Some of you may remember the old PC game Giants: Citizen Kabuto:



In this third person shooter you'd play as one of three characters whereas each one of them had a different playstyle and different gameplay (one guy had weapons and a jetpack, then there was a women with spells and other powers and one giant monster). As far as I remember each level was at least one island that was completely free to explore and move on but of course you always had to fulfill one or more goals.
I think a game like this could be a nice project to work on, maybe even with a co-op campaign, if you can manage to come up with a nice story and/ or good characters.

EDIT: http://en.wikipedia.org/wiki/Giants:_Citizen_Kabuto