I like this idea a lot. Keep it in it's own battle arena in the city, if the player isn't online, auto drop listed winner every 12 hours or something.
Wouldn't have any negatives on the city, especially if was in its own instance. Could be anything from a housing (once implemented) as a "reward" or a title for keeping it x time.
I see nothing but positives if done correctly
Gamer tag: DasPanzerKat NA Xbox One
1300+ CP Battleground PvP'er