With 11 players happening I think there may be too many roles.
I like the mafia having additional roles, and the vote changer is a great role...and the Role preventer is an interesting power as well, because it gives the mafia a chance to cancel the Santa.
However, I think Santa should be limited to how many roles he can give out. It is OK to give him the chance to give out different roles, but perhaps limited to 2 a night or 1 a night. Also, I would consider giving the vote changing role not to the mafia, but to Santa to give out. With Santa giving roles out and not knowing who players are...the mafia has a high chance of stealing roles for themselves, which means they don't need as many roles themselves for the game.
So I think limiting the roles Santa can give out to 2 roles, and then giving him the option to give 2 of 4 powers out, investigator, Doctor, Vote Changer, Vigilante could be interesting. The Role Canceling power is Mafia's counter power.
Then if enough players sign up give us a Killer abominable snowman role, that plays like the T-Rex. That would seem to add the most balance to the game.