I already talk about it a bit in the general Newcomer thread, but here I'm going to do a long post about every fighter on this theory and my thoughts on them:
-Monster hunter: I think it's one of the most likely fighter and for being one of the biggest Japanese franchise popular worlwide, I think it could have been consider for base game. I do wonder if the moveset planned for the Hunter was indeed given for Byleth (I think a leaker imply this or something). If that's the case, I don't think that the Hunter is coming.
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Shantae: I don't really see her spirits as evidence that she was planned. It is a bit weird that she's the only third party with an online icon with no playable representation, but I think that's not a really big evidence to think that she will be playable. I still think her chances are low because some of the most popular indies (Sans and Cuphead) got a Mii, so I would be pretty surprised to see her playable.
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Rayman: I'm not sure if Rayman was really considered. My guess is that Ubisoft really wants him in, but Nintendo doesn't because he's not popular enough in Japan. The Ubisoft Mii that we had in wave 5 also hurt his chances I would say.
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Koei Tecmo: It's hard to tell if a Koei tecmo character was really planned, but I feel like Hayabusa is still one of the most likely character, so even without this theory I think it doesn't really change.
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Lloyd and Yuri: I do wonder, if they where really planned for base game why are they not in fighter pass 1? Seems kind of weird, although I think they can still happen, but I have a feeling that Lloyd Mii might come back with Pyra/Mythra so we'll see.
-Katalina: Hitagi confirmed that
she's not coming to Smash after some time. Was probably a mistake on Hitagi's side and I think she was never planned.
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Decidueye, Alucard and Chun-li: Seems like they where replaced (by Incineroar, Belmont and Ken respectively) so I doubt they will be back.
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Rhythm heaven: Could happen if Sakurai has more liberty and still wants them, but unsure.
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Ayumi: I think the game localisation was decided way too late (considering that when it was announced, it was Japan exclusive). Beside, Sakurai consider her for Melee (20 years ago) so things might changed for him (more than Chorus kids which was only 6 years ago for Smash 4). Again, could maybe happen if Sakurai was given more liberty, but I don't see it if Nintendo choose the characters.
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Sans/Underatle rep: I think Sans' reaction was way too late and the fighters was most likely decided by that point.
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Isaac: Could happen if Nintendo really wants to please the fan, but with the franchise being dead (and with Camelot working on another sports game) I'm unsure.
So in summary, in my opinion here's how I would place each characters:
-Pretty likely: Monster hunter, Lloyd/Yuri, Hayabusa/Other Koei tecmo rep.
-Could happen if the starts align: Rhythm heaven, Ayumi, Isaac
-Probably won't happen but might have slim chances: Shantae, Chun-li (considering that she's iconic and there's almost no content from modern Street fihgter), Sans (but the timing would be very small for that to happen)
-I personally don't see it: Katalina, Decidueye, Alucard