Plain and simple, there haven't been any Wii U Pokémon games as of yet besides Rumble U and Smash 4 (and Pinball RS but that's a VC game), and now that the Wii U is entering it's third year I wouldn't be surprised to see one at least announced this year.
Whatever it is, it has to be something that
1. Is interesting enough for a standalone game. The Wii U needs all it can get.
2. provides us with something the 3DS Pokemon games can't.
Sure, Pokemon Snap U would work very well, but perhaps if they tweaked the gameplay a bit so it was a bit less on-rails? Like, an open-world Pokemon Snap, where you can just wander around the Pokemon world and use the Gamepad to interact with wild Pokemon somehow.
Pokemon Colosseum 3 would also work. As long as there's no Shadow Pokemon/snagging. I know it was the defining feature, but it was really boring and got frustrating. Just a Pokemon game with a story that doesn't involve gym badges will do well.