I’m not so sure. While they have a low winrate, I suspect a lot of players enjoy playing them and want to play killers. And there is something to be said for the people who play an important role being those who enjoy it rather than people who are annoyed and just trying to get through the game to play something else. Even if that is a problem, it could just use the method ToS does with its picks - basically, the roles in the game are chosen first, then the game tries to assign people to them based on their preferences (so if everyone hates on a particular role, or everyone love a specific role, it will cancel out and have no effect at all rather than changing the chance of specific roles appearing.)
What I would probably do, though, is to let people blacklist one or two roles they don’t want to play as. Then, we can automatically collect data on the most blacklisted roles and discuss if they need to be tweaked to make them more fun (I suspect that you’d see a ton of people blacklisting the Court Wizard.) Again, this would use the method described above, so it wouldn’t affect the frequency with which roles appear, only who gets assigned to them.