I have always played the rooftop running bow wielding assassin the the ES games...
So the dark brother hood has always been my favorite part of the game.
Going back to Oblivion I would love skulking on the roof tops with my bow and pretending to assassinate a certain NPC, and way back then i thought to my self...WOW this would be so cool if my target was a real person...
SO! here is my idea..
Outside doing the the planned dark brotherhood quest line.
players should have access to a quest to preform the "The Dark Sacrament"
This will let the player select their enemy they want the dark brotherhood to assassinate. That player who has killed you several times in PVP that you want to get back at or what have you.
After completing the dark sacrament this person is then put on the list of people who need to be assassinated, thus those of us in the dark brotherhood can take the contract and seek out the player to assassinate. After doing so they get their reward.
The way this could work is possibly when you select the contract it gives you a letter where the person was last seen in the world and a marker on the map of a around about (not the exact location but a large circle on the map) where this player is (and it constantly updates)
Also when the player who's target is assassinated they get a letter with the black hand to let them know their target was assassinated.