Let me expand on this idea a bit...
How about, if you are in a lowbie zone associated with a particular city, and you are KOS to the guards & citizens of that city, you are flagged such that a player who has visited the bounty hunter and paid a fee (another way to remove plat from the game) can now attack you, and kill you for a bounty. This way, a fee is paid for the right to hunt the KOS player, and only by accomplishing the mission can you collect your reward.
Also... in order to prevent exploiting for plat.... Any player killed for a bounty would incurr debt - and the price required to pay off this debt would be *equal* to the bounty paid for that player being killed. This way 2 people cant conspire to let one kill the other, and gain plat.
The fee to request the right to hunt a player & the bounty paid could scale upward with the level of the KOS player.
Example:
Joe talks to the bounty hunter about being interested in helping eliminate the Most Wanted in Erudin. Bounty hunter offers Joe the chance to go after Steve. Steve is a level 50 player. Joe needs to give the bounty hunter 5 plat for the right to hunt Steve, and can only hunt Steve until he either turns in Steve's head, or sees the bounty hunter to cancel/switch assignments (the 5 plat fee is not returned). Later that day, Joe happens to see Steve in Mistwoods, and kills him. The next time Joe hails the bounty hunter, he is rewarded 55 plat. Joe's net profit is 50 plat. If Steve decides to visit a healer to pay off his debt, it would cost him 50 plat. Multiple players could pay 5 plat for the rights to kill one player, but the reward would need to be divided equally among them. Also, once a person has been killed in this manner, they cannot be hunted again until they get to the next level (or in the case of a level 65, when they earn another AA point).
If you hail the bounty hunter, and the person you are supposed to kill has been killed by someone else, you have the ability to take a new target without paying the fee.
I know, it seems like this would take a lot of coding. Maybe that time would be better spent elsewhere, but I just figured I would brainstorm with this. Got nothing to lose!