I really hope they don't and are kept a one time thing in 8 Deluxe. Not only would it be weird for these non Mario characters to keep re-appearing in Mario Kart, but it won't be as special anymore if they do come back. This is a new Mario Kart game, there's really no reason to focus on anyone but Mario characters, especially if this is a launch title.
This is actually a good topic... IDK how I didn't think of this haha. Personally, I don't think so, but would be cool!
Hot take: I think Shulk is more likely than Kirby.
I really hope they don't and are kept a one time thing in 8 Deluxe. Not only would it be weird for these non Mario characters to keep re-appearing in Mario Kart, but it won't be as special anymore if they do come back. This is a new Mario Kart game, there's really no reason to focus on anyone but Mario characters, especially if this is a launch title.