Mario Kart 8

[Kriven]

When you say "Mario game," do you mean an official title or a spinoff? If you mean an official title, then Donkey Kong hasn't been appearing all that much in official Mario games ever since his (well, actually Cranky's) arcade days. Sure, there's the Mario vs. Donkey Kong games (including the first one), but I think of those more as spinoffs than anything else.

I honestly see the Mario vs. Donkey Kong games as sequels to the original Donkey Kong trilogy... in a way, they're more like the original Mario franchise line, and DKC and SMB are the spin-offs.