Nintendo needs to do a Mario/Donkey Kong adventure game. They've assimilated the worlds of both enough in sports crossovers, it's time to take it to the level we all love. Rather than spending loads of time and money on another Mario adventure, and another lackluster Donkey Kong spinoff, why not bring the two together in an adventure starring both stars?
Mario, DK, and pals team up to take down both Bowser and K. Rool? Epic.
I see it as a Sonic Adventure 2 type game, where you have the Mario storyline and the DK storyline, yet both interweave. You can play as Mario, Luigi, Peach, or Yoshi from the Mario storyline, and Donkey Kong, Diddy Kong, Dixie Kong, and Funky Kong from the DK storyline. All play entirely differently and are used to complete different types of stages, but ultimately, you are granted the freedom to complete and and all stages with any character.
EDIT: Or of course, just limit it to Mario and DK as playable characters. That would still be amazing.