Street Level Superheroes - Been Done Already?

Perhaps you could add a twist:
one of the heroes (only the player will know this) is actually a supervillian! The bad guy can still save people, etc in order to preserve their cover, but there will be things they don't need to do or can do differently.
With the super villian running the scene secretly, the game could be given more structure, kind of like a malevolent GM out to get players in a RPG only nobody knows who this person is.

I like the fact that superpowers (the Superman, Green Lantern sort) are very rare or absent from the game and that most of the super heroing is done Defendor style with a trench club in the back alley.

I must admit I'm not a real connoisseur of comic books and superheroes, but it seems to me that much of the real attraction of them to the fans is the relationships, rivalries, grudges and feuds between the characters, and this is something that I haven't seen done in board games before. Sure, you might have a certain villain gunning for a certain hero, or a hero may have a dependent to protect, but I've not seen a game where this evolves over a number of plays. It's the sort of thing that works well in roleplaying games, though.

So, if you manage to have these relationships between characters building and developing over the course of a number of plays, you could end up with something that is very different to other games and thematic gold!