I think the Wii U and PS4 will be pretty neck and neck when it comes to sales, but if Nintendo's software isn't pushing system sales then it will end up falling behind the PS4 - around ~3 - 3.5 million.
I don't see the Xbox One generating too many sales because of the negativity still lingering over it, and its higher price compared to the other two consoles. If Microsoft winds up reversing the negative stigma surrounding their console then maybe it'll get 2.5 million at best, but I don't see it going past 2 million, and even if it does it'll have a difficult time getting there.
I'd like to see one on the Wii U too, but it probably won't happen. Game Freak has stated multiple times that the main Pokemon games are meant for handhelds and it will stay that way.