A 3D mario like Galaxy won't be a system seller. Heck Super Mario Galaxy did like 10 million while New Super Mario bros Wii did like 24 million.
A proper 2D mario has better chances to be a "system seller". A proper one of course, New Super Mario Bros U is good but it could have been WAY better.
"I've Underestimated the Horse Power from Mario Kart 8, I'll Never Doubt the WiiU's Engine Again"