Don't worry, there will be a special challenge for Legendary Wizards that will arrive at the same time as Wintertusk.
Oh, I'm not worried! I'm trying to figure out how it'll start. One of the huge problems I've had in the 1.5 years I've been playing is getting the mission notification at level up. They don't always come, or I get notified of missions that don't exist. If we aren't going to trigger a "level up" situation (because we're already maxed), how are we going to be able to start it?