I also think the second screen of the wii u was horrifically underrated,
During the switch build up, they said there may in future be backwards compatability with old Nintendo controllers, and I really hope they mean the game pad.
That would open up the wii u back catalogue for those who didn't adopt the console, add functionality for future games (Map / inventory screens, think fallout 4 virtual boy) as well as the entire catalogue of DS, 3ds and new 3ds games.
With the joycons there should be no need to go back and add compatibility to wii motes, all that tech is there already, and with more buttons!
It would be great however to give those wii u owners and unexpected boon, as well as birth another extra peripheral for switch owners to buy, even if it were a smaller screen / controller.