I own NSMBU and would be fine if it got ported. I never bought Luigi U, only played it at a friend's house. I'd be down for re-buying because I like the Switch platform more and would rather replay it there. It also gives those who didn't have a Wii U a chance to play it.
For those who are like, "This is dumb, they should make a new game," I disagree. I honestly don't understand how locking the game to DOA console benefits Nintendo or the audience in any way. If you already own the game and don't want the port then don't get it and you're no worse off. The game came out 6 years ago, can you really have buyer's remorse after all that time? If you're a Switch owner who hasn't played it before, yay, you can play the thing now. If you're Nintendo, yay, more money for not much work.
Ports are done by small teams or external studios so they could easily port it while also working on a new one anyway.
If it goes ahead then I'd like to see some new content, whether it's new levels or game modes. Female playable characters (Peach, Rosalina, Toadette, etc) would be very welcome because two Toads is bloody ridiculous. Online and LAN play would be neat but I wouldn't expect either.