I would absolutely love another Diddy Kong Racing. Throw in Dixie, King K Rool and a bunch of forgotten characters and I'd be pretty happy.
Really though aside from the design of the game what I'd really love to see is the return of Winky, Zinger, Expresso and more! I know Retro wanted to do more with animal buddies but why didn't they?!
Ground vehicles would include - Rambi, Expresso, that mouton goat from Jungle Beat, Squitter and what ever else.
Water Vehicles to act as the hover craft would include - Winky, Engaurd and Orco that Killer Whale from Jungle Beat.
Flying vehicles would include Zinger(I know, but they were going to be used that way in Donkey Kong Racing for Gamecube), Squaks, and *sigh* Parry the Parallel Bird.
Seeing those older animal buddies back in action seems like it would spark more interest in the developers to bring them back in the main line DK series.