Here's my two cents:
Adventure mode: Eh, nah. Racing plots are usually non-existent or totally lame. I'd rather just focus on the racing.
Different vehicles: This has potential definitely, but how would it work with the existing MK items. My favourite vehicle in DKR was the hovercraft, just hooning around and doing donuts in battle was fun, so if any made it in, it deserves to be that!
More tracks: That's a given, but there has got to be a limit to it. To spout a random number is easy for you guys, but the developers have got to set limits. And with the tradition of four Mario Kart Cups (Mushroom - Special) it was always going to be 16.
Tougher tracks: Nah, the tracks weren't what makes DD hard, it was the AI. Trying to defend a lead is very tough in 150CC (and lets not comment on All Cups Tour , so why make it impossible for you?
And don't rip on Baby Park - that is one of the most challenging courses in the game. Try playing a four-player versus, and making it through the minefield of shells and bananas before you tell me that it sucks. Just because you keep turning in the same direction doesn't make it any simpler.