I really liked the Wii U version. It was actually the best of the 3 NSMB games i played (didn't play the 3DS game).
But when i tried to play it a second time, it was really hard for me to play it again. On the other hand, i had a blast replaying Mario 3D World.
The style of the game is just unnatractive. It really, REALLY needs a full overhaul.
There's a few things Nintendo can try:
They can go back to the 16-bit style - as in, go retro; they can also really make something new: forget playing with adult Mario, give me a teen like Mario and Luigi in a more stylised world.
Using a more kid/teen Mario would be something that Nintendo hasn't tried and it could help set the ambience for a different attitude for all characters, to give the story more relevance and also the relationship between Mario and Peach.
