The thing about Mario Kart 64 is that is perfected the formula. The blue turtle shell was perfect in that game. The multiple shells, and holding a shell back for defense everything was perfect...so even with cheating AI and giving last place better items, it was pretty balanced.
That all changed after the blue turtle became explosive, and became worse with the more outlandish special weapons. I think Nintendo needs to go back to the original formula, and just make a new Mario Kart following the exact same formula of the Mario Kart 64, but adding better tracks, HD graphics, simplify the weapons. Keep the drifting and boosts from the other games, and perhaps even the hang glider stuff, but make the weapons balanced again.