...could be made with the Assassin's Creed engine?
I think it would be badass!
Yes, AC got very repetitive, but this can definitely be fixed. It had an 'investigation' system that could be expanded for Bats. He could investigate all manners of crimes. Some would just be muggings that he stumbles upon, others could be assigned by Gordon when you see the Bat-signal in the sky. You could have a crime meter. If you ignore a lot of the lower crimes, Gotham becomes worse and worse, but it wouldn't necessarily effect getting through the game, you would just get a different ending.
It's got an open world feel that could definitely be tweaked. Rather than several cities, just make one huge Gotham! Bats can wall-climb, and you can add a zip line, a Batmobile or cycle instead of the horse, and it would be awesome getting around Gotham.
Finally, throw Nightwing and Robin in as playable characters with different movesets and slightly different equipment and I think you have a game that could sell a lot.
HJJOUNB SAWLWEN HATES MEO!!!! -Royal