I think that N64 Mario should make a comeback in the spinoffs.
Just going by these offical renders, there's enough of a distinct personality between the two. 64 Mario looks like he just burned your house down for the fun of it while Modern Mario looks like he's your standard nice guy.
They could easily present 64 Mario as a younger Mario (I mean we already got Baby Mario). Even if it's just Mario Kart Tour, I'd be pleased.
I feel the example art here is actually one of the friendlier-looking renders of N64 Mario. I always thought the N64 Mario had more of an attitude, maybe just due to how the silicon graphics presented themselves, but I can't help it.