Nintendo has recently filed a trademark application for the Nintendo 64 controller in Europe, which given the timing might well indicate that a Nintendo 64 Classic Mini could be in the works for next year.
Such trademark applications have also been filed for the NES controller and the SNES controller, so this logic does seem sound given that we now have a NES Classic Mini and a Super NES Classic Edition is on the way later this year. The 'goods and services' part of the application references physical goods, too, which has boosted hopes further.
What games would you like to see on a Nintendo 64 Classic Mini if this was to come to pass? Let your imagination run wild with a comment below.