mario kart, smash and amiibo are evergreen products, they will keep the momentum. add variety of exclusives and a likely price cut, wii u should peak in 2015. 4 to 5 million.
last direct showed that 3ds will also have a great library in 2015, new hardware and amiibo exclusive to new also helps.
id assume it will be flat.
So, combined, 14M+