Well, Mario Kart 8 was originally a Wii U title, so I think it's a definite possibility. Backwards compatability with controllers was also added much later in the Wii U's lifespan as well, with the Gamecube controller peripheral being released alongside Smash Bros. for Wii U. It's not impossible to do these things.
Not only that, but a lot of Wii U titles could also be played on the gamepad only, so there is no reason they couldn't have Switch ports. It might be a bit less convenient for some games that used both the gamepad and the TV screen, and titles like Starfox Zero would obviously need some tweaking, but...yeah, I'd expect to see more in the future. I remember when rumours were flying around the Switch about it being a hybrid - which turned out to be true - that some Wii U ports (I forget which, but at least Xenoblade Chronicles X) were being planned for the Switch, too. Wouldn't surprise me if those were true.
Historically speaking it's likely, too - we had N64 game on the Gamecube, Gamecube games on the Wii, and Wii games on the Wii U.