This way we can have F-Zero pilots/machines on Mario Kart 8 tracks and Mario Kart characters on F-Zero tracks! Both games have zero gravity and F-Zero machines and/or pilots with Mario Kart items would be hilarious! Think of the integration like Left 4 Dead 2 and its predecessor. The game would be new and cost 60$ for retail and $50 digital if you have Mario Kart 8. But again, you get to have integration of F-Zero and Mario Kart if you have both games!
Also, Nintendo can have a Download F-Zero DLC demo on Mario Kart 8 menu screen as another way to advertise the game.
And of course you can buy the game separately as well just like NSLU.