I'm an idiot and think I locked myself out of the "You're My Hero" achievement (the one for completing all challenges.) I cleared all the takeover stuff, and now there's no rival gangs around to get the "kill 50 gang specialists" challenges... nobody roaming around, no more gang ops, and even all the random survival modes I get calls about seem to be only for police.
Looks like I'm stuck doing another playthrough to finish those up. :-( I wish this game had a "New Game+" or at least the ability to replay story missions.
If you're going for an S-rank, don't be a dummy like me! Do the challenges first!