That begs the question. If only 1-2 characters were planned originally, then what about the rest of the "missing" mii costumes? We're missing Chocobo, Geno, Heihachi, Lloyd, Gil, Rathalos, and Hunter. All those indicate 3 companies, but originally the plan was to just have 1-2 characters as a bonus. If we only ended up getting 1 fighter, would they just cram all the remaining Mii costumes into there? Or would they just not bother with bringing them back, and focus on creating new costumes based on the series of that character.
A new Namco character feels inevitable (90%+ likely) at this point, they're missing 3 costumes from 3 entirely different series. If they got no character, then they could have just moved Heihachi to the Terry mii costumes. Heihachi or Lloyd make sense, but it could end up being someone completely different. Capcom and Square, I'm a little bit mixed on both of them.
Capcom: We're missing 2 costumes, but in the end it's just 1 series. Resident Evil is one of their biggest series, but they were fine with them getting spirited. Monster Hunter had an assist in base, but I don't really think that counts them out anymore. So really, we have Monster Hunter, DMC, and Phoenix Wright as their biggest potential series at this point. There are also smaller but inactive/unrequested series like Okami, Darkstalkers, and Viewtiful Joe. However, I could see the Monster Hunter costumes return with another companies character, and DMC and AA get spirited.
Square-Enix: I think a rep from them is pretty likely (not as much as Bamco, but more then Capcom). The DQ costumes didn't contain Chocobo/Geno, which is a big omission. However, as others have brought up; we did only end up getting DQ/Enix costumes alongside Hero, which may explain the lack of Square costumes (perhaps the 2 sides of the company are distinct from each other like Activision Blizzard). Due to all the things supporting him, Geno is the front-runner here, but the competition is pretty fierce comparatively to Capcom/Bamco. You have characters like Sephiroth, Tifa, Lara Croft, 2B, BD characters, and a potential Sora. Again, I lean on a playable Square character but a potential costume/costumes coming in with another Square Rep could change things completely.
I've heard people say that Square will not pair up their own property with other 3rd party characters; but I don't actually know where it came from? Sure, it hasn't happened yet, but we've only ever had 2 Square characters; which really isn't enough evidence to throw out the idea completely. I do think it's unlikely, but getting a Square costume with a Namco/Nintendo/Capcom character is completely within the realm of possibility. That said, I still thinking getting another Square fighter is more likely then them getting costumed alongside a different companies character.
Yeah this too. If let's say ARMS was character 6 and Lloyd/Heihachi were character 7, then the we should have had all the mii costumes in by then; obviously things changed since then, but if that were the case it's possible that something like the ARMS fighter was originally going to be a large pack of missing costumes.
However, this depends on the assumption that the first characters that are negotiated for end up coming first in the fighters pass. It's also possible that they get the rights to the character/content of each company first and then decide on the order of release later (meaning that there's a sick twisted timeline where Geno ends up as fighter 11 instead of 7 or 8). However, with ARMS being 1st party; it also messes with the rule a bit, it's possible that the 1-2 bonus characters would always contain ARMS + 1 more.
Since both Byleth and Banjo Kazooie (two heavily Nintendo associated characters) contained 0 costumes from their own series, but rather costumes from random 3rd/1st party titles; it's completely possible the ARMS character has costumes completely unrelated mii costumes and potentially even some returning costumes (hopefully not Geno).
EDIT: Another potential thing that struck me while I was writing this is that Sakurai just said that "Originally, 1-2 more characters were planned". But he never states that he had a clear idea of who those characters were at the time, it may have been a situation where he agreed to work on more characters, but the characters themselves were not planned at the time. Even if he knew those 2 characters and 1 of them was from ARMS, wouldn't it make more sense for the 2nd character to be Rex and Pyra compared to Geno/Bamco. Sakurai did say that he wanted ARMS and Rex in base, and with ARMS being 1 of those characters there's more precedence for Rex being the 2nd one. While I dislike Rex, he was definitely more requested then an ARMS character and is one of the most requested "promotions" especially in Japan. I doubt that the order of release is the order they get the rights to a character, but if it is that could mean that Rex is potentially character 7.