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.
What makes you think either of those characters would be in the series at all? There's no way they'd get Gary Oldman for more than a guest shot in the pilot if they're lucky. As for Blake, he was a uniformed officer in TDKR, set 8 years after TDK. If the series is set closer to TDK, Blake would probably be a rookie at most, maybe not even enrolled in the police force yet.
It's more likely the series would focus on other police characters -- maybe the Gotham Central
cast including Crispus Allen and Renee Montoya, maybe Harvey Bullock, maybe a whole new set of original characters a la Agents of SHIELD
. Maybe they'd get Matthew Modine to reprise his role from time to time -- he's probably more affordable than Oldman.
None of the villains can be too dangerous or else Batman would come out of retirement to help out.
I've debunked this two or three times already. He wasn't in any physical or mental condition to come casually "help out." It took a terrorist threat, a looming invasion of the city by a whole army, to bring him out of his self-imposed isolation, and even then he needed technological help to compensate for his physical impairment.
Also, nothing too shocking can happen to change the status quo because it has to fit within the continuity of The Dark Knight Rises.
How is that different from any other TV series? I don't recall Law & Order
doing anything too shocking to change the status quo in New York City, since they were pretending to be in the real world and thus couldn't diverge too far from what was really happening. You can tell plenty of crime stories without blowing up the city.