That's fair, actually. I just haven't seen a weird role in GAFia that made me go "what is this?" so I'm not sure if the rule is necessary. If we want to encourage traditional roles, I'm all for that. My concern was that it'd lead to people "checking the list" to see if a role claim is possible. At that point it's just a semi-open set-up.
They erased people from a game!
I'll share an anecdote from the first game (which was also my first forum game, and my first "mafia" themed game). It was also my first closed setup game.
While we were playing it, I would do some googling into mafia roles (I'm sure others did, too). You know, trying to guess what people might be based on what typical mafia roles are. We got sidetracked with the whole "I send owls out" thing that was a bunch of bullshit; I never really found anything like that anyway. I did find the Switcher, though, and that did seem more plausible.
Eventually, I claimed Doctor, a fairly common role that happened to be the weird red herring of the game...but that was a very simple and clever play by Crab on the expectation of those standardized roles.
So the anecdote is that I don't think working with the assumption of the common roles is a bad thing at all.
I'm not against deviating from the core set of roles, but my concern is that people wont even start there, and just make a ton of weird, broken stuff (that the mentors then have to go back and reign in).
I would rather people start from vanilla roles and push out from there, than somehow to try make an insane thing work. Tethering role claims to something isn't a bad thing, IMO. Otherwise, we're going to get really bizarre stuff in the near future.
"Bro, I'm a 5 shot Drunk Child who turns Mafia when he witnesses a crime, but also has wings, like that of an angel but also of a demon"
"Uhh...is that legit?"
"I dont' know, let's talk about it for 20 pages"