They don't have to be direct ports ... like I said with Smash I'd recommend strongly remaking the other Smash titles too or including content from them to have one huge central Smash experience.
For Splatoon, they could easily add a lot of new content like new maps, new weapons, new play modes, maybe even a new single player campaign or a new local multiplayer mode.
Zelda is a new game, so basically they can treat it as such. I think they should take advantage of the new hardware here and enhance the graphics at least (unlike Twilight Princess).
Mario Maker could add a lot of new content and features too.
I don't think just taking a bunch of Wii U games dumping them on the new console is the greatest idea though. They should put more effort into it.
