As others have mentioned, Mario Kart 9 is almost certainly a launch title or at least launch window. I feel the game will have a bigger focus on competitive online gameplay and challenge leaderboards.
Some form of new IP to show off whatever the console is capable off. I'd love to see a Nintendo Land 2 (they should make it part of the system and release dlc just before a new release, free for online + subscribers, to promote the games.) It's more likely that they've been experimenting with roguelike/lite and survival games though, as those genre's are booming right now. Give it the old Nintendo twist.
A revival of an old IP. Something along the lines of Kid Icarus, F-Zero, Duck Hunt or another go at Star Fox. This will depend on what direction the NextNintendo will go. Hopefully they'll do something with all the 3D tech they have.
A sequel to a relatively new IP, perhaps Ever Oasis or Astral Chain.
A remaster/remake of a pre-gamecube title. Something along the lines of Link's Awakening.
Luigi's Mansion might release the first Halloween the system goes through and Mario Maker 3 will be inside launch window. Not sure about 2D Mario as I think we'll see some sort of 2.5D Mario around the release of the movie.