To be honest, Mario Kart 7 is so far (i think) the best incarnation of Mario Kart since the Snes days. Don't get me wrong, it does have a few flaws. But it is a great racer. I like the new underwater sections, the gliding and the new tracks. The choice of Retro tracks, some shouldn't be there really (i'm looking at the Wii tracks). They should have added a few more tracks from the original MK, MK-64, MK-DD and MK-DS. Were there 7 Mario Kart game, i can only really think of 6, which are
Super Mario Kart
Mario Kart 64
Mario Kart Double Dash
Mario Kart DS
Mario Kart Wii
Mario Kart 3DS (or 7)
Can't really think of what the 7th title is to be honest. As i mentioned before, they could re-vamp the retro tracks a little to freshen them up a little, bring them into the modern era as it were. They could add water sections like Wave Race, turning cars into boats as such and racing across the water. Or have water-based tracks. Just a thought to be fair.