The more I think about it, I could actually see King Boo as the next challenger.
Beyond just Luigi's Mansion 3's suspicious absence from spirit events, it wouldn't surprise me if King Boo was originally considered for FP5 as the 2019 Nintendo release rep, but was beat out by Byleth due to Three Houses having better timing. Perhaps Sakurai really liked the idea and decided to put King Boo into the second Fighters' Pass? It would be a variation on the second chances theory, since this is a cut FP1 character rather than a cut base game character, but it would still fit the general theme.
Plus, ghost powers, tongue-fu, and a portrait final smash would be pretty badass.
That being said, the /v/ leak is 99% fake and, even if by some miracle King Boo does happen, that was almost assuredly a lucky guess.