The issue with first party speculation for Me Personally is that, at this point, the first party inclusions I'd really want are almost always characters who Nintendo themselves would have no reason to pick. As in, usually from franchises that died off before the Switch was even a thing, never mind having had a game on it. When the first party among my wants that has the highest shot of getting picked by Nintendo is Waluigi, and the others aren't even close, that's saying...quite a bit.
Like, from franchises already on the roster, I'd only really be super into additional characters from Super Mario (namely Waluigi), Pokémon (depending on the choice), Fire Emblem, Punch-Out!!, and ARMS, but for characters from new franchises?
Give me Muddy Mole, Takamaru, Starfy, Henry Fleming, Saki, Kyle Hyde, Mach Rider, the Chorus Kids, Sukapon...there are so many great characters in Nintendo's backlog.