I had a thought. What if a new DLC pack like New Super Luigi U gets made every year, with large new sets of levels of varying difficulty and different gimmicks? One with new or returning powerups? One that emphasizes Yoshi? One that makes the playable cast that of SMB2/SM3DW with according strengths/weaknesses? A Wario Land-inspired one? It'd be like the e-Reader cards for SMA4, only more likely to succeed and more potential for awesome.
This would solve the complaint that there are too many NSMBs while not even slowing down on the delivery of fresh Mario to play