“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.” - Masataka Takemoto, director
It's certainly not something that will be implemented in New Super Mario Bros. U, but it seems that the idea might resurface in a future Mario title down the road.