My biggest potential problem with this series isn't that it's another cop show (although that's one of them), but that it's a prequel. This means that Gordon and Blake absolutely cannot die. None of the villains can be too dangerous or else Batman would come out of retirement to help out. Also, nothing too shocking can happen to change the status quo because it has to fit within the continuity of The Dark Knight Rises.
A better idea would be to set the series after The Dark Knight Rises. Batman is dead (as far as people know), Harvey Dent has been revealed to be a fraud, and Gotham is in shambles. So much more potential for future story-telling without the limits inherent with a prequel.