My hunch has been that the next 2D Mario will be the big "system seller" holiday title for Nov/Dec 2019. I expect Smash (obviously), Pokemon, and Animal Crossing will have already come out before then, either later this year or first half of next year.
I also expect they will follow some of the approaches in Mario Odyssey's 2D sections to change it up and not have it be NSMB Switch.