Well, there hasn't been a single third party guest inclusion in any of Nintendo's usual Mario spinoff titles that weren't developed by another developer outside of Nintendo. Mario Tennis, Mario Golf, Mario Party, Mario Baseball, Mario Strikers, and Mario Kart have not had a single third party guest in all of their releases across 20 years of consoles. Not to mention, roster isn't usually such a big deal with any of these outside of what characters are missing from the usual crowds of Mario extras that star in the game, so there's practically no incentive for Nintendo to go through a process like that and invest in a character whose fan base isn't even particularly focused on them and when people only minimally complain about those rosters (Mario Kart 8 got some flack for its roster of racers, but it barely even registered for most people in my experience and racers generally don't bring the variety that a Smash character does for example).
The exceptions to this rule have been: Mario and Sonic series (which explains itself and is developed by Sega), Mario Hoops 3 on 3 (Developed specifically by Square Enix and contained Ninja, White Mage, Black Mage, Cactuar, and Moogle), Mario Sports Mix (Developed by Square Enix again and contained the five previously listed gusts from Hoops and Slime), Fortune Street (Square Enix developed and a bunch of Dragon Quest characters), and then the Mario Kart Arcade releases (Which were specifically developed by Bandai Namco and had Pac-Man in them).
What we see when we put all of that together is that Nintendo has exclusively stuck to their first party IPs when developing their own Mario spinoff titles. The only times we get third party crossover is when another company is involved in the development of the games. Funnily enough, it's usually been Square Enix responsible for those titles, so there might be a better chance at them working on another Mario project in the future which would be ideal. But I am saying that Nintendo does not operate around third party crossovers in their own Mario spinoff titles. That can certainly change in the future, but they've pretty well stuck to that across all the major releases in recent memory. That's why stuff like campaigning for Geno in Mario Kart or Mario Golf doesn't really make a whole lot of sense to me. There's the third party barrier that has to be broken first all around before we can talk about Geno having a chance in those games, and then specifically bringing Square Enix to the table as the next step. Even the recent DLC offerings for Mario Kart 8 and Mario Tennis Aces absolutely echo Nintendo first in those choices. I'm more than willing to acknowledge a possibility once any of these internally developed spinoffs get a third party, but for now, they just don't.
