6. Martian Manhunter
A Martian Manhunter sandbox game could be a dark and gritty Men in Black meets Superman, again free roaming the city, but with an air of stealth and "behind closed doors" themes. For anyone who doesn't know who this guy is, he's a member of the Justice League of America (JLA) and for anyone who's never seen this guy in action, think of him as Superman but able to shapeshift into other peoples forms, amongst other things. Maybe for this game once again you're defending the big city, but instead of cruising around in costume and flying over the heads of cheering fans, you're fighting a group of Martian aliens that are hiding in plain sight, and are trying to take down Planet Earth from the inside out and as the Martian Manhunter its your job to move around the city kicking villains arses back to whatever crumby planet they came from, but doing it quietly. You would be able to explore the city at will, finding certain collectibles and taking on side missions, and for the most part everything would look normal, but instead of the typical "oh, hunt down this guy before he mugs someone in an alley" maybe a developer could switch it up a bit and instead you could take on the form of a civilian, hiding in plain sight in the crowded streets, using your "vision" to pick out other evil aliens doing the same. You spot one, you follow them, you stop them zapping a poor old lady in an alley way with their ray gun (that already sounds 50 shades of evil), and all of a sudden theres a super speed rooftop chase between you and some mutant alien wacko? But because you took it to the rooftops and used stealth, theres no panic on the streets that two aliens are fighting. And that would be another mechanic, kind of a form of GTA's "wanted" level, but instead level 1 could be "the public know nothing" to level 6 being "the government knows you're here, expect tanks." Think Prototype 2 for an idea.