Don't know...
If we will see an exclusive GTA for PS4, this GTA could reach 30M.
Then there is Halo 5, but I don't think a Halo game can sell more than 13M in this generation.
And maybe we will see a suprise hit this generation? it is too early to answer your question.
All the same, I guess Mario Kart 7 can't outsell Pokemon X/Y.