Please feel free to leave constructive criticism or additions to the topic :-)
I would really love for there to be a police DLC. I'm thinking the ability to patrol the streets in both marked and unmarked vehicles. Of course whilst patrolling you won't be able to acquire a wanted level, and you'll be able to get backup units out for traffic stops, possible drug deals and so on! You should be able to customize a majority of vehicles with unmarked police kits (perhaps 3-5 different ones for variety), placing hidden lights all around the car. Also, since the radio is non-functioning in police vehicles (at least in SP), you could use the radio wheel as a control panel for various lights and sirens. The perhaps you could arrange buy-busts, where you would go to locations where narcotics known to be distributed and you could buy them as an undercover, call in that the deal has gone through and then take down the dealer. Of course there would be several ways for this to go wrong also. Maybe an increase in traffic offenses by NPC cars (running lights, stop signs, etc) giving you a reason to pull them over (or start a pursuit) and then writing them a ticket, or possibly discover that the person was in possession of drugs, drug paraphernelia, driving while intoxicated and so on and so forth. These are just the things I could come up with off the top of my head, but think of the endless possibilities, and with the right execution of this, personally I probably could spend hours upon hours upon hours detaining criminals and serving justice in Los Santon and Blaine County, and I know quite a few people with whom I've discussed this, that share my opinion =)
As far actual gameplay missions as a cop goes, there could be a string of missions where you often have to make choices as to whether you want to be a officer of the law or if you want to be corrupt, those choices affecting your possibilities, e.g. if you through an extended period of time act corrupt, your coworkers might loose faith in you and you'll become more or less infamous in the eyes of the Los Santos population, but on the other hand you'll have more influence with the criminal underworld of Los Santon and vice versa.
And unlike a DLC containing new missions, or perhaps one or more new protagonist (which, of course, would also be great), a DLC like I described could keep you entertained for an unlimited period of time, instead of just doing the new missions and then go back on a killing spree like you used to.
Those are just my 0.02$ - I hope you would consider putting this suggestion in your Top 5 Video, perhaps bringing this, or a similar idea, some publicity :-) And I hope you enjoyed reading my suggestion :-) Pardon any grammar mistakes, not my first language!