I think reserving crossover stuff for DLC is a smart way to go.
On another note, your post made me realize how much I’d appreciate “themed cups.” What's the point of having the "Triforce Cup" if we just go to Hyrule Castle and then a bunch of random Mario locations? I feel like no one's really concerned with "immersion" in Mario Kart, but it really is weird that they never even attempt solid theming when it comes to cups, aside from like...Mushroom is easy, Special always has Rainbow Road, etc.
But why not a Shine Sprite Cup with Delfino Airstrip, Delfino Plaza, Gelato Beach, Corona Mountain? Or a "Barrel Cup" with DK Mountain, Mine Cart Madness, Fruity Factory, Gangplank Galleon. Maybe people would miss the diversity that every cup would bring, but it's easy to make a Sunshine, Galaxy, Yoshi, DK, etc. theme that still offers plenty of unique locations. I feel like this could make the cups feel more meaningful and distinct, while only slightly restricting the variety each cup could bring.