I mean, Mother's content in Smash is already one of the most absolutely wild things ever. Two playable unique fighters, four completely distinct and unique stages, full 3D models of additional party members in Final Smashes, several items, several Spirits, and a lot of music. Plus Brawl even had a Porky Boss fight. All for a series that only has three games, only one of which ever made it to the US originally in a failed marketing campaign, none of which made it to Europe originally, only to finally get a couple re-releases in the 2010s, but only for the much less beloved first game to finally get an official release instead of the beloved third game. It's also a series that is officially over and the creator has zero desire to return to, so Nintendo can't really do anything else with it outside of remakes/re-releases.
I'm not saying it has too much content, but I do want people to seriously think about how incredibly unlikely of a reality that is. Much more successful franchises with more releases and their own considerable impact don't have playable fighters from Nintendo, but this incredibly unlikely franchise currently has way more content than series like Xenoblade and Punch-Out. You could always add more content, but I think it's pretty undeniable that Mother fans are eating extremely good in Smash and always have in general.