The Syndicate

As a host, I always try to come up with lots of new mechanics, so its hard to choose just one. I would probably say ability types that work off of a players position on the player list. I have had it so an ability split the role list making those on the top half unable to target those on the bottom and vice versa, kills that could also hit the player above/below the selected target and even an entire mafia faction bonus that revolved around the player list being divided into zones and the members of that mafia choosing which zone they inhabit. Players who weren't in the same zone as that mafia player had the effects of their abilities halved against them and several of the roles in that mafia had area control abilities, like switching up action priority in the night time for players in her zone.

Creating independent roles in general is my greatest passion in mafia though.