Oddly enough, I was thinking along the same lines myself.
It'd be nice to be able to get one rival to declare war on another, with a hostage to ensure good behavior. Or to take (or offer) a hostage as part of peace negotiations. There's one "nice" way for players to win - negotiate alliances among the ruling gangs so everyone can live harmoniously together. Of course, for your own hostages to mean something, you'd need some sort of bond there. That would work well with the proposed marriage objective. Or with the PC's offspring, although it might not work so well if he has the little treasures wearing a collar and working on their backs.
You could also, potentially, improve your street cred by taking the mistress of a rival and publicly humiliating her. It'd be instant vendetta, of course, but sometimes it might just be worth it
