User blog comment:Jimeee/Skyrim 1.9 beta update adds Legendary difficulty and raises level cap (Updated Apr 8th)/@comment-24.210.90.170-20130315041801/@comment-4709478-20130317182827

Dragonborn is not the last DLC.

As the rumored "Redguard" DLC should be comfirmed sometime in the future. If not, then it'll be something else, because they've said they aren't done with DLCs when DB was released.