What COULD save the Wii U? It's pretty simple

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Metroid Prime 4 - Obvious why

Animal Crossing U - This would definitely help boost a lot of units

Yokai-Watch U - Now THIS would save the Wii U in Japan at least. Like it would sell pretty damn amazing. (Yokai-Watch is selling better than Pokemon in Japan. Not sure if it will sell great in the west, but it would boost Wii U in Japan)

Pokemon Red/Blue HD remake - This would boost sales practically everywhere

Nintendo making a deal with Capcom to release an exclusive Wii U Monster Hunter. If it was Monster Hunter 5, it would no doubt boost a ton. Especially in Japan.

 

EDIT: I am not asking for what is possible, what won't happen, what might be possible etc. 

I am not asking if it would be better for Nintendo to save these for the Wii U's successor or not. 

All I am saying with this post is what the Wii U needs in order to be saved. Regardless of probability.