Sandbox is my thought. If we can create huge, immersive environments like, say, the Spiderman games (not a fan of them, but the city is HUGE). For the available morphs, run something like out of Dead Rising, where the individual morphs aren't always immediately apparent, but have an orange icon highlighting them (small morphs or what not). I think that choosing how you complete a mission, and not necessarily having a hard order to things (have a mission map a la GTA, but still maintain a generally cohesive storyline). And I firmly believe that multiplayer would be a must.