I sort of like the idea of Shigeru Miyamoto lording it over his Nintendo minions, upending tea tables, making people deathly afraid to add new Mario powerups, but it turns out that, with Mario Kart 7 for the 3DS, Miyamoto was more generous, allowing the team to move the game into areas he didn't necessarily approve of.
This is especially the case with Kart customisation, which has been added into Mario Kart 7. Miyamoto isn't a fan, but is happy for the team to experiment.
"I was actually pretty well against some of the customization features of the game, though," said Miyamoto in a Famitsu interview translated by 1up. "It can be fun to win money for racing and use it to buy parts and such, but I didn't think that had much to do with the core fun of the series. The idea for that came from the studio staff, though, and my final response was 'If you can build this customization on top of a solid control and gameplay foundation, then go ahead.'"
According to Miyamoto, there was pressure for him to pay more attention to the game's development, but the core of Mario Kart was so solid that he felt content to leave things alone.
"Sometimes people yelled at me to look at things more closely, but like I said, the core of Mario Kart is pretty solid at this point and I think it's safe to have it evolve in a pretty staid and traditional manner."
Having played a fair bit of Mario Kart 7, I can confirm that the game remains true to previous entries in the franchise. The customisation is mild, but interesting enough to encourage tinkering. Personally I like the feature.
What impressed me most, however, was the game's track design, which is completely out there, pushing the boundaries. Mario Kart DS was probably my favourite game on the handheld, and my favourite game in the series, so I'm very keen to see the final version.