MK's biggest problem is longevity. Its far too easy to see everything the game has to offer in a few days, if not hours... so more tracks and cups are a necessity. 6 cups, four tracks each.... and I can't see why MK can't do an 'excite' and require a little more than placing in the top 3 or 4 to progress.
I think you could easily pop a story in the game - or at least explain why the racers are racing. Motivation, perhaps one character has to rank above a rival - Mario MUST beat Bowser, etc. Each character could then have a cinematic ending, providing a reason to continue to play.
Controls... lets do something radical. Steering with the nunchuk control stick, while flicking the wiimote will fire off your weapons. A is hop, powerslide by tilting the wiimote and nunchuk. B perhaps a taunt and item stop.
I'd also like to see less cheaty weapons. Why not have 3 coloured blocks... a selection of 4 weapons per colour Red are POWERFUL but heavy items (and slow you down greatly), Yellow are medium in both weight and power, and slow you down only a little. Greens have no effect on your speed but are less powerful. This would help stop gamers, holding back on tripple red shells from 8th place.