When recently interviewed by magazine GamesTM, New Super Mario Bros. U director Masataka Takemoto was asked directly whether level editing might show up as a possibility in future Mario games:
“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.”
In the Coin Battle Mode on the Wii U, coin placement is as good as it gets in term of editing a level… but it seems like a first step. And now you know the possibility is out there, and it may appear someday in the future.