Interesting question - I presume you mean a true, cop orientated version of something like GTA vs games like Crackdown and the like.
Clearly, the general market for this stuff isn't interested. The majority of gamers clearly want to be the bad buy, to be Tony Montana, etc. and behave badly vs play a game where they're supposed to be stopping such behavior.
It would be interesting to see, no doubt about it though.
It's not open world, but it'll be interesting to see response to Heavy Rain where it appears you play as a cop and some 'normal' joe's vs the usual hero or bad guy.
Try to be reasonable... its easier than you think...