Super Smash Kart (MK style game, but with all Nintendo franchises).
Tears of the Kingdom (not that I think it won't come out for Switch this May, just think they'll also port it if there's no backwards compatibility)
Nintendo Land Style Minigame collection- I think Nintendo Land was a great idea. Not something to build a launch around, but a good supporting concept if done right.
New 3D Mario Game- Hard to believe it has been over five years since Odyssey. Seems like the right time to get a new one, and launch time would be the perfect time.
Smash Ultimater- I don't expect a ground up new Smash game, but I can see a port of ultimate with about 10 new characters+all the DLC repackaged as a launch title, hopefully with some better net code.
Street Fighter VI- With this non-exclusive this time around, I can see Nintendo having this as one of their big third party gets.
Call of Duty- We know it's going to be on Nintendo platforms, so hitting the next system at launch seems like a smart move.
Kirby and the Forgotten Land 2- A little soon, but I think they're going to want to capitalize on the success to turn Kirby into an A-tier franchise for them.
Not an exhaustive list, and I don't necessarily think all of those games will be there. But those are some I think are likely.