MK9 for Switch makes no sense. They should hold off until the next console. They are still selling buttloads of 8 that they would canabolize.
If they release Diddy Kong Racing 2 it would need to be vastly differentiated.
I guess tires rules out Starfox racing.
What makes no sense is leaving tons of money on the table that would also please a majority of the fanbase. I don't think Nintendo is going to ignore the fact that far more fans like Mario Kart than any other racer just cause a port already exist on the system. A 20+ million seller like MK9, even if it were to cut MK8D's legs to 0 (lol no), would still net a lot more profit than the 2-3 million sales Diddy Kong/F-Zero/Kirby Air Ride would generate. From both a consumer and business standpoint, MK9 should hold more priority than most other games let alone just racers.