From an IGN interview
“We’re always challenging ourselves to create something new, so hopefully you’ll see a new kind of Mario in about a year or two,” Shigeru Miyamoto said in an interview with IGN.
“Maybe next E3 we’ll be able to share something.“
Miyamoto explained that the challenges Nintendo faces in tweaking the Mario formula are unique from those dealt with in producing Breath of the Wild because of how wide an audience Mario games reach.
“It’s kind of difficult with Mario because some of the more important conventions of Mario are based on the approachability and accessibility of those games,” Miyamoto said, noting that having new people join Nintendo has allowed them to look “at the conventions with fresh eyes.”
In terms of development time, Nintendo usually requires around 3 years to make a new Mario game, 3 years since 3D World would end this November so we should be seeing it soon, or at least at next year’s e3