This is a good question....I mean, I seriously don't know which one. It ain't Zelda or Mario (Mario already has Super Mario Party), Metroid Prime 4 is certainly not it either, Pokemon Let's Go is coming out this year and we already know that, Fire Emblem is next year, Donkey Kong had Tropical Freeze on Switch this, Bayonetta 3 is next year, Kirby got a game this year....
The only franchises I can think off that haven't had a release or announcement have been Kid Icarus, Star Fox, Mother and F-Zero.
....I hope it's the last one.