I was looking into the Justice System idea, and was thinking to myself, with the Justice System I think they COULD implement a Thieves Guild. And with players getting bounties put on their heads by the guards / cities, they could also implement that into the Fighters Guild Bounty Hunting.
Allow for PVP bounties where the players go after other players that have outstanding bounties with their faction, or even other factions, adding a mercenary feel to it that the guilds are supposed to have anyway, with them being non-faction specific.
I do understand that there could be issues where v14 characters have the highest bounties, or a v14 player gets a bounty on a lower level character, to counter that they could put in level restrictions with hunting other players only allowing say a -2/+2 level difference, or even the War system of leveling you to your target if you are far below them, or to high above them.