I think it would be nice to get a Mario RPG character into SSB, but I don't want Geno and I definitely don't want Paper Mario (ugh, a third Mario, no thanks)
Fawful would be the best. He is cute and charming and silly while also being a spiteful weirdo. He has major roles in two Mario & Luigi games plus a minor role in another one, and is a fan favorite character.
I guess he'd also work as a boss fight, seeing as his chances of being playable are null, but making him playable would be so perfect. He is such a silly character and there's a lot of fun to be had with his attacks and presentation. Despite being a fan favorite character and being the best character to represent Mario RPGs, he has no traction for SSB and would never be made playable now that the Mario & Luigi games might be done for good. But if Sakurai knew how to make the best ssb game ever, he would add fawful