Nintendo announced a Luigi-themed DLC pack for New Super Mario Bros. U in today's Nintendo Direct. The DLC, titled 'New Super Luigi U,' applies changes to the main campaign to make every single course "all-new", essentially allowing for a new Luigi campaign.
The DLC is coming this year. Nintendo president Satoru Iwata said the content was "large-scale," similar to that of a new game, and will therefore take some time to develop.