A trademark doesn't necessarily mean confirmation of a project, but I can easily see Nintendo following through on this pair they filed this month. Japanese Nintendo uncovered two trademarks: "Mario vs. Donkey Kong" and "Spirit Tracks," which were apparently filed for "program for home video game machine, downloadable video game program, and program for smartphone" details.
This all-encompassing listing could mean a lot of things. Nintendo might be gearing up for a Spirit Tracks re-release on 3DS for E3. It makes perfect sense: it's not a super high-risk venture for a system that's becoming increasingly lower priority. Mario vs. Donkey Kong has also enjoyed a long life since 2004, hosting games on the GBA, DS, 3DS and Wii U. A Switch iteration with amiibo support (like amiibo challenge) is a no-brainer.
Of course, either would translate very well on mobile devices. Mario vs. Donkey Kong already lends itself well to a touch-based control scheme and Spirit Tracks is a perfectly accessible encapsulation of what makes Zelda great: a modified mobile edition could bring in fans of all ages.
Spirit Tracks Trademark [Japanese Nintendo]