The first and biggest rule should be FMs are informed of all characters going into their faction, and do have a right to say 'nay' to things. They can't 'approve' the character but they can pause the approval for faction inconsistencies. In the last year I've seen instance of someone evade the FM and try to make a character for a faction they were banned from, and I've also seen someone make a character for a role in a faction that literally says on the character creation page "Speak to the FM before taking this role" without speaking to the FM. Both of these are just plain unacceptable.
Most of the rules though would be quality control rules, and steps the GM needs to follow when checking the character over. Like ensuring physical description of player characters is more than just a single sentence for each line. We don't need master works of literature there, but as has been said frequently "The description needs to be enough that someone drawing your character can get a pretty good idea of what they look like only using that."
As for enforcement that would be a bit more difficult, but generally it would be when people find characters that were approved without following these rules, they can inform someone and the staff can do something about it. Granted the punishments would have to be very tactfully done cause we don't want the players in the plot to suffer because the GM is lazy, but be don't want the GM to not care about the rules either. Maybe things like temporarily closing the plot to new members, increasing the frequency of their plot reviews for a time, having them have to go through a 'test' to make sure they do know the rules, things like that.
Also just an add on, I would like to point out that a huge chunk of the problems that have appeared with characters since I was here were caught by Aendri, and a lot (I wouldn't say most cause I don't know the numbers exactly) wouldn't have been fixed if it was not perceived that as a character approver he had a higher veto power than the GM. So that Character Approve banner was doing -something-.