I was debating on bringing this up or not but since we have been speculating the next fighter is female I thought it would be okay.
So the newest credible rumour is that a female character is next. This guy predicted DQ Hero correctly even before Erdrick was ever speculated/leaked and the most important part is that he guessed correctly that Hero has no female alts.
He's now been posting in English instead of Japanese sometimes... possibly cause his tweets have now gained traction in the west.
Here are some of the tweets from the account with his posts which were before and right after Sakurai finalized dlc with Nintendo.
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October 2018
Translation:
"I have a friend irl saw the development of Smash Ultimate, and he said that they're not making DLC yet statement was a lie.
I wasn't listening to him well but he predicted Isabelle so he has credibility.
He said Akira Toriyama art style character(s) and a female fighting game character? are coming.
I don't really care about them just give me Neptune."
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December 2018
Translation:
Nintendo Switch VR will be released.
Dragon Quest protagonist (excluding women) and a female fighting game character (I can't remember who) will be added to Smash Bros.
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June 2019
And here is where it gets super interesting. He came back and now says the character is "like a female fighting game character"
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people have been heavily speculating Reimu and he denies that today
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There could be the possibility KOS-MOS is that female especially if we are talking Namco. When I think "like" a fighting game character I think of lots of moves and similarities. KOS-MOS is similar to Alisa Bosconovitch in Tekken for example. It's all speculation but I believe this rumour is pretty credible. Also, KOS-MOS appeared in Soulcalibur III as a CAS... not necessarily a fighter but it's something.