I won't deny that a Mario Kart based movie would be interesting to see; however, I just don't see the possibility for one anytime soon. So far, almost all Mario based animations (including the film, cartoons, anime) have all had their fair share of negative criticism and never became too popular on their own. The closest thing to a Mario based element in a film I can think of right now is Bowser's appearance in the upcoming Wreck it Ralph movie (even then Nintendo supposedly wouldn't allow the movie makers to use the Mario Bros. for even a cameo without a large sum of money - I guess the original Mario film really put them in negative terms with Mario on the big screen).
Regardless, that's not to say that the Mario series may not see a movie or animated series in the near future (the series is getting more and more popular by the minute!). In my opinion, Mario Kart could be an element used in a potential Mario based film; however, I don't see too much of a plot made out of Mario Kart alone. Something made out of the whole Mario series as a whole (in an accurate way and not like the way the original horrid movie did) would be great to see.