Bad idea. There's a reason individual players have a ruler character in more than one realm - other people either choose not to run against them in elections because they don't want to be a ruler; or, they can't be bothered to write letters and campaign during said elections; or, if they become rulers, they don't put in any effort to try to make their realm (and the game) more interesting. There are numerous realms across the game where people could virtually walk in to ruler (or other government) position if they put in the slightest bit of effort.
Hard restrictions of this sort never work in this game. How would people feel if everyone was forced to play one priest character to revive the religious game, for example? The game works best when it's as open as possible.
