You know I find this idea really interesting. Every batman game has you playing as the batman. Some do it well, some do it poorly, but most have similar mechanics, similar means of attack, similar motivations and similar weapons.
I like the idea that instead of playing as the dark knight you play as some low level dipshit on the city streets. Caught up in the same tide of theatrics that caused many of Batman's own villains.
Another interesting aspect, that I really hope the developers pick up on, is that batman and the Joker would have no qualms whatsoever beating the shit out of the players, regardless of which side they are on. Batman because they running around panicking people with guns, Joker because he's a damn lunatic.
Hell, why stop with batman and the Joker, why not include other bosses to serve as their henchmen, Mr. Freeze? Poison Ivy? Scarecrow? Why not?
On the whole this seems like an opportunity to see a different side of Gotham, what Gotham looks like before batman shows up, a bloody and violent mess where crazy people dressed like hamsters run around with sub-machine guns trying to steal hilariously poorly defended jewels for Gerbiletee, queen of the domesticated rodents, so she can use it as a focus for her death laser.
Bonus points if they get Mark Hamil in to do the voice of the Joker.