If the new Mario Kart includes elements of Diddy Kong racing, it would be legendary, but it seems very unlikely.
If Diddy Kong elements mean story or something like that, it's not unique to it. Other games have story mode. The true element would be different vehicles. I've wanted Mario Kart to have Jet-Skis and combine Waverace and MK. I don't believe Mario Kart would ever have planes.