KingofPhantoms
The Spook Factor
The latter has happened before with King Goomba/Goomboss and the Star Spirits from the first Paper Mario game, but both of their 3D debuts were a long time ago, in Mario games themselves rather than crossovers, and, IIRC, none of them have shown up in more than three games at most....Now that I think of it, do you think if they added a Paper Mario partner, they'd keep them papery, or would they give 'em a full 3D model?
It's entirely possible, but it's also possible that they would want to keep the Paper Mario style and aesthetic for any playable Partner characters like they want to keep the style and aesthetic of Mr. Game and Watch. It would all depend on the developers' vision and what they consider important or most functional or practical for a 3D fighting game if any of them are ever taken into consideration.
Personally, I'd like to see them portrayed in 3D, in no small part because I'd like to see most of the Paper Mario Partners in mainline Mario games, but I'd be fine if they remained 2D. I'd just be happy that they got into a Smash game at all.