So what's the difference between Mario Kart + Other Nintendo cameos and Nintendo Kart then?
All the guest characters in Mario Kart 8 came with courses based on their home series - it is the expectation. People would have been pretty disappointed if Link's course wasn't clearly Zelda themed but was instead another typical Mario Kart course with his name slapped on it like Link's Bowling Alley.
The beauty of the Mario universe is there are no expectation of what locations they can use so th epossibilities are endless. Toad owns a factory apparently, Luigi an airport and Waluigi has a giant pinball machine somewhere in this world.
I'm open to Mario Kart having guest racers and courses. I enjoyed them in 8, but I don't want them to come at the expense of the originality.
The difference is design around it from the outset, which includes crossover tracks in the main cups interspersed with everything else, perhaps adding more cups like they did with the DLC but right off the hop. This opens up other opportunities and expansion of ideas already implemented like SFX variation (like you hear when item roulette is active in certain courses), item variation beyond the boost currency (coins/rupees/bells), which they could implement by course or by character, while keeping certain commonalities with mild variation
if they chose to go that route. It's a difference between intentional design choices vs. add-on/after-thought, as well as it being all-in right away rather than this incremental creep that's already been happening which will possibly (I'd almost say inevitably) bring the series to the same conclusion some day in the future due to positive consumer reception. Preferably before I succumb to old age and its requisite frailties.
In addition to character-themed courses (one of which was a Battle Mode course), we also got Excitebike, 2 F-Zero tracks and a callback to how crossover has been actually been happening since MK7, with the Wuhu Town Battle Mode course being remade.
And you say "all the guest characters" like there was this huge mass of them... as of Deluxe, it was 4 of them, and their respective gender clones.
And yeah, if anything gets sacrificed in such a transition, it'd probably be that there are a bit fewer retro courses, and there's nothing more original than not dipping into the well of past MK games as much.