I'm honestly not a fan of adding characters for the sake of meeting some sort of quota. If a character gets it in, it should be on their merits as a character. I like characters for who they are, not what they are.
That said, if we're on the subject then these are possible contenders that I'd personally like to see:
Tethu - If Ever Oasis does well enough. I hope it does, but it doesn't seem to be getting much attention sadly. The little guy is too cute to ignore imo.
Anthony Higgs - IF and only if they do more with him in the future. The only new Other M character that I liked, but they still didn't do much of anything with him. I like the fact that he's close to Samus, so hopefully they bring him back and flesh him out. So long as it doesn't involve making it an Other M 2...
Doc Louis - I'm still very supportive of Doc Louis, even if it's as a clone/semi-clone of Mac. He's Little Mac's mentor and taught him everything he knows, so I think he could work. What I'd like even more though is for a new Punch-Out!! that follows Doc in his prime on his road to becoming the world champion.
Twintelle - If ARMS does well, I think Spring Man is more or less a given in the future. Once he's accounted for however, she already seems like a popular choice, though I'd still say it's too early to really tell given that these characters were only just recently revealed and the game isn't even out yet. We'll have to see who remains popular after the game has been out for a while.
Plenty more females that are actually plausible as well: Viridi, Medusa, Elma, Daisy, Isabelle, Ashley, insert assorted FE characters (Anna, Azura, Lyn, etc.), Inklings (who count as both of course), Nikki, Krystal, Lip and Ayumi as retro reps, etc, etc. We go over them pretty often.
As I said though, I'm not going to support a character on the basis that they're female or "x" attribute. They need to be good characters like all the ones I listed above (well, those are good imo at least
)That just seems silly to me and doesn't do the character in question justice if all they are to you is a check box to mark off for the sake of diversity.