The thing that separated Diddy Kong Racing from Mario Kart 64 was the "Adventure Mode" aspect, of having a storyline and characters and an overworld to explore. Of course, DKR's implementation of all that was rather perfunctory as a result of it being bumped up for release to patch over the delays of Ocarina of Time and Banjo-Kazooie, but if Monster Games/Retro were to actually flesh those aspects out? It could be something distinct and complementary to Mario Kart 8.
Of course, there's a snowball's chance in Hailfire Peaks (Fire Side) that this Kevin Callahan rumor will pan out, but... oh man, if it did? We could be looking at an E3 with Yooka-Laylee, Banjo Threeie, Donkey Kong 64 Twoish, and a new Diddy Kong Racing. It would be all of DKvine's dreams coming true at once.