3D Mario seems to make the most sense, mostly because of the 6 year, soon to be 7 year gap.
Mario Kart 9 or 10 or whatever they will call it will be a year 2 release. They'll want a bigger install base and they'll be able to use it to get a post launch bump (or mountain). Same for Smash, and let's be honest, it's possible there won't be another Smash title. Not likely, but possible.
Metroid Prime 4 does seem to make sense, but it could also just get a Deluxe treatment a year later, but I don't see Nintendo doing that.
I think a Donkey Kong 3D game could be the Mario Odyssey of the Switch 2.
And there will definitely be a quirky game like Arms (could be Arms 2) or 1-2 Switch that takes advantage of one of the unique features of the system.
I'm really curious what third parties are cooking. There's no way they are going to be caught flat footed this time around. I expect quite a bit of "this is what Nintendo fans like" content hitting the launch window. Whether they'll get it right will have to be seen. Just look out for the next Rabbids Kingdom Battle (with Nintendo character tie-ins or not).