It would be great to give riders at levels 100+ the opportunity to ride some routes that no one else has access to. Maybe a couple of routes in each world?
Maybe 100+ gets the Galibier and Sarenne climbs that link up to Alpe du Zwift.
And give those routes really high xp bonuses for completion. Just what every level 100 rider needs
Private Island
I also suggested a private course only available if your ride streak hits 52 weeks.