I don't think so. Mario Kart 7 is currently over 2million behind Pokemon X and Y.
Last year it "only" sold 1.3mil. Assuming similar drops to last year it will take it over 2 years to pass current X and Y sales.
X and Y is actually outselling Mario Kart 7 so far in 2015 so the lead will only continue growing for the near future.
The 3ds is already 4 years out and a successor will surely come out within 3 years or less. There just isn't enough time for mario kart 7 to catch up to X and Y even if X and Y sales collapse after this year.
There is really no other game in competition for best selling 3ds game unless they start heavily bundling super mario 3d world again. And any new games that come out on the 3ds will not have enough time to sell as well as pokemon x and y. The first pokemon on a nintendo handheld has always been the best selling one.
I think a Wii U game is out of the question considering how poor its sales are so the next question is if there will be a ps4 or xb1 exclusive that can outsell pokemon x and y. The best selling xbox 360 exclusive is Kinect Adventures at 21.4mil or ignoring pack ins, Halo 3 at 12mil. The best selling PS3 exclusive is Gran Turismo 5 at 10.6mil. Considering Pokemon X and Y will sell at the minimum 14mil lifetime the only way a ps4/xb1 exclusive will beat it is if it becomes a bundled title for over a year. So I think Pokemon X and Y will mostly likely be the best selling exclusive of this gen.
