In an interview with GamesTM magazine, Masataka Takemoto, the director of New Super Mario Bros. U was asked about the possibility of user created Mario stages.
He had this to say:
I think the Coin Edit feature in Coin Battle is a step closer to that. There is a possibility of this happening in the future, but there’s no use just blindly making that feature available. We would need to carefully think about how to make such a system work. The important thing is that it has to be something that is uniquely Nintendo. Where a beginner can easily make something and have a good time, and simultaneously, someone who is really into Mario can delve into it deeper and make something more complex. So that is a challenge we will need to tackle in the future.
Currently Coin Battle Mode is the closest players can get to creating a level, allowing users to play coins within levels. A level editor would be perfect for New Super Mario Bros. U, giving players even more play-time out of their favorite game.
Being able to share levels and compete for the communities favorites would add something more to the game.