I think one of the smaller albeit very notable problems with Mario Kart Crossroads relates to two specific characters: Waluigi and Daisy.
Fans of these two characters have been campaigning for them to get more major roles for decades, and for a period there was legitimate worry Nintendo would phase them out (especially in Daisy's case). If Mario Kart becomes a crossover there is, depending on the roster size and Mario-to-crossover ratio, a decent chance they'll lose their undeniable most defining roles. If the contagious crossover disease spreads to Mario spin-offs, they could face extinction.
Fans of these two characters have been campaigning for them to get more major roles for decades, and for a period there was legitimate worry Nintendo would phase them out (especially in Daisy's case). If Mario Kart becomes a crossover there is, depending on the roster size and Mario-to-crossover ratio, a decent chance they'll lose their undeniable most defining roles. If the contagious crossover disease spreads to Mario spin-offs, they could face extinction.