Here's a thought. They can't use the Wii branding anymore and we haven't seen any Wii Sports, Wii Fit etc. type games. But why not use the Mii branding instead? It works just as well and they all utilize Miis anyway.
I think after the Wii U flop Nintendo realized trying to cater to casuals isn't working anymore so they went in the completely opposite direction. The closest thing to a casual game Nintendo has come out with on the Switch AFAIK is Ring Fit, but from what I've seen of that game, it gets hard. Although you can adjust the difficulty. But it's far more complex of a game than any of the Wii series of games so it doesn't really fit in, it's its own thing.
But ehh... I can't really say I miss those games. Wii Sports (and Resort) were fun for a bit but there was not much replay value for me at least, and all the others were meh. And the other games Miis were utilized in, they didn't really fit in with the rest of the game. And they're not exactly appealing looking anyway.
The whole Mii system could use a redesign to be more detaled, and also give devs some freedom to style them to fit in with the individual games' art styles. Since we now know Breath of the Wild uses modified Miis, that shows what they could accomplish with them. I wouldn't mind Miis if they were more detailed and fit in with the style of the game better.