This ended up being a pretty lengthy read, sorry!
So yeah, the Mii Costumes. I've been thinking a lot about this too, so I'll give my two cents. As we know there are five Mii Costumes left, effectively representing four characters. And people only care about three of those characters, since I think it's fair to say Chocobo will not be playable. Chocobo does have a lot to do with my personal theory though, so they're still important and I'll get back to that.
I do want to say, it's no coincidence that the likes of Heihachi, Lloyd, MH and Geno - arguably the four most popular returning Mii Costumes - have stayed absent until FP2. Perhaps they were all considered possible candidates for DLC and held off during the discussion? That's personally how I see it. But anyway...
___
I'll start with Lloyd, who I believe is next on the chopping block. With Heihachi / Min Min and now Gil / Steve, it seems that the Namco costumes are currently being trickled out similarly to Capcom in FP1. Whether or not they'll stop before we get to Lloyd is yet to be seen, but given the current trend it's just not looking great for him. Not to mention, it's a little odd that they would release Namco characters in this fashion for FP2 if they had more content coming down the line. I wouldn't be particularly shocked to see him, but there isn't a whole lot in his favor at the moment. Not to mention that Yuri provides pretty solid competition for him either way, so the hypothetical Tales character might not even be him.
___
Monster Hunter on the other hand... I think they're worth keeping an eye on. As I mentioned before, Capcom costumes can drop alongside any character, but it's worth mentioning that we haven't had any in FP2 yet (old or new). I actually believe these costumes will return either way, since they're just generic armor sets, but of course if they show up without a MH character we can count it out. Monster Hunter is a massive series and one that has grown substantially in the last few years - Monster Hunter World has become Capcom's biggest game ever, and the series has two Switch exclusive titles on the horizon. Seems like a pretty safe bet, but Rathalos already being a boss shakes things up a little bit.
Personally I don't think this matters. Monster Hunter's situation is quite different than it was when (presumably) Rathalos was negotiated for the base game, and with Sakurai's previous statements on ARMS (Spring Man is an AT, too) plus the suggestion that Steve was discussed for nearly five years it's not unreasonable to consider the possibility of MH content being revisited or expanded upon. So in a way, I actually think Rathalos being here is a good thing. It means these discussions have already happened before, and there's no doubt plenty of room for MH's representation to grow. I'd be surprised if a MH character was never at least a passing thought, and there's more reason to pull the trigger on that now than ever.
I think Monster Hunter is pretty likely. There are no "locks" at this point and even my most confident predictions are more like 50/50... but MH is one of the characters I have the closest eye on right now.
___
And last but not least, the infamous Geno. I'll just put it bluntly, I think Geno will be playable. Obviously yes, he's got a good deal of fan demand in his camp and the Smash 4 costume / spirit prove that he can't be so difficult to negotiate for, but my reasoning has more to do with the way costumes have been released thus far.
Both Geno and Chocobo are currently missing. This puts Square Enix in the very unique spot of having absolutely none of its returning costumes released so far. I want to emphasize something that we all know - Square Enix is weird. While these costumes could appear in the second half of FP2 just as easily as Namco's didn't appear until Min Min, I find myself thinking back to Hero's wave of Mii Costumes. During Hero's presentation I was 100% ready to see Geno's costume return, but to my surprise it was only DQ costumes - and not even a full wave, Wave 2 weirdly only having four costumes with no returning costumes for fluff either.
There are several ways to take this, one being that Geno and Chocobo were pulled from that lineup for an unknown reason, as their presence would have led to an average 6 costumes - maybe this is when the planning stages for FP2 were in full swing, or potentially the negotiations themselves? This is likely the point where FP2 plans would have formally begun, soon after Nintendo was at E3 brushing shoulders with some key names in the industry. More than that though, I have this itching feeling that SE costumes will only come with SE characters. And regardless of whether it's Geno or not, I think that we'll get a new SE character in this pass with Chocobo and / or Geno as a costume in that pass. It's theory-craft, but more than anything I think it's awfully fishy that Capcom and Namco have had their Miis released incrementally when Square Enix didn't even see them drop alongside their own character.
I do want to acknowledge the possibility of Chocobo not returning because of Final Fantasy f**kery, but that could go either way in my eyes. Geno's costume will return (or get a makeover) or he will be playable, I'd be shocked if he didn't appear in any capacity given that he's able to at least have a spirit in the game. I foresee him being the hardcore fan pleasing character a la Banjo.
So in conclusion and TLDR, I'm not like super duper confident in anything right now (except maybe Hayabusa?) but Geno and Monster Hunter are easily on my shortlist of characters that I believe have a great shot of being playable according to the current costume release trends and just being a huge series with its foot already in the door respectively.