Lower Gangs could announce bounties on NPCs of opposite factions in a brute way through the beggar system. As you start to favor a gang more than others, you could have stuff happen, like random bandits of the rival gang to appear and gank you Morrowind Assassin Style.
It could be like this:
::[ Message Relayed by Messenger Girl : "The Silver Brand is looking for John Nickel after he kidnapped Goru's girl. Bring the bastard to us whole and get your purse heavier" ]::
The rival could also randomly send something like that:
::[ Message Relayed by Messenger Girl : "The Sharkhide will pay if you give Gorus Slow a good spanking and convince him that Alicia is not interested anymore. She got on John Nickel a real man. Try to be gentle" ]::
I like the sound of this, but I don't agree with automating your "favor" with the different Lower factions, since I think that stuff works better in DM hands.
Seeing bounties from the Lower gangs would be awesome, and allow Bounty Hunter concepts to exist outside of Upper!