I think the Super Mario-series should take three different routes...
1) A Super Mario 2D series
Games such as Super Mario Bros., Super Mario Bros 2, Super Mario Bros. 3, Super Mario World, Super Mario World 2: Yoshi's Island, Super Mario Land, Super Mario Land 2: Six Golden Coins, New Super Mario Bros., New Super Mario Bros U, New Super Mario Land 2, Super Mario Maker etc
These games are easily accessed, and attracts casual and main stream gamers, of all age groups, as well as hardcore fans of the Nintendo universe.
2) A Super Mario 3D series
Games such as Super Mario 64, Super Mario Sunshine, Super Mario Galaxy and Super Mario Galaxy 2.
These games are not as accessible as the 2D games. They are mainly for hard core fans.
3) A Super Mario Semi 3D series
Games such as Super Mario 3D Land, Super Mario 3D Land 2.
These games are quite accessible, but with visuals matching those of the real 3D series.
I believe Nintendo should try to release at least one in each of these series for every hardware plattform. For Wii U we already have a 2D game and a semi 3D game. I think we need a true 3D Mario Game aswell before the Wii U fades away.