At least Rayman has the excuse of being nearly impossible to make a faithful Mii costume out of. I'm going into some heavy speculation about Sakurai's philosophies and Ubisoft's policies, but I think it could just be that when Sakurai thinks Rayman, he thinks the very first Rayman game, but Ubisoft would prefer that any representation of Rayman is the modern Legends/Origins style. Often times people pin the blame on Sega for a similar reasoning behind why Sonic doesn't change color to pay homage to his friends or a powerup like Super Sonic, but other than that I can't think of any proof that a third party has outright kept their character out of the game over design conflict or that Sakurai has turned down a character for that reason. But, even Rayman's spirit is his modern design, despite the fact that many spirits aim for the oldest piece of artwork they could get a high quality scan of.
Sorry if I came off seeming to think that you believe Geno is only coming back as a costume - that's not the case at all. What I'm hoping you'll take away from this is that there is no "premium" costume, and if there were, Geno would not be getting one. Sans got an "insanely faithful costume" because how the hell would they botch it? I seriously want someone to come up with some way that Sans could get a costume and it not be faithful.
I understand that to you, "faithful" is the issue, but you need to understand that these costumes aren't all made to be faithful, some of them are made for this really rare concept called "fun".
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Is this the best Nintendo could do to make K.K. Slider into a Mii hat? Of course not. They could have given him the full head treatment. But they didn't, because they didn't want to. It's funny to wear the K.K. Slider hoodie. This may not be the entire reason, but it's an attempt to illustrate the point that a decision is made by someone on how to represent the character, and sometimes they go with a comical approach. I mean, the full costume has K.K. Slider shooting lasers from the tip of his guitar... Why
wouldn't they have some fun with that?
Another thing to keep in mind is that there's almost no way the original designers or a designer from the home franchise of these costumes isn't consulted on the design of the costume, and even if they aren't, Toby Fox
was consulted on the design of the Sans costume. I guess that he could have told them "the body is fine, let's just ignore the head", but the thing about costumes with a visible face barring costumes like K.K. that are intentionally designed to be funny, the character has a natural skin tone. There is no bone white for Mii fighters, and I don't think they would have ever expected us to do something like that.
I drew some ****ty mock-ups of the alternative Sans costumes that would have been "unfaithful"; let's say he gets the K.K. Slider treatment:
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Horrifying. Also he kind of looks like Papyrus. That's not very representative of Sans. How about a hat:
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Well, it's better! But maybe it could be confused for a Team Skull hat? Or to be more realistic, it's just the top of a skeleton head. At that point, you're really treating Sans like a meme - any skeleton could be Sans and Sans could be any skeleton.
Is this working against my point? Are you going to tell me "well since they
could have done that it means that they put extra effort into Sans!" But that means that since they didn't do this for Black Knight, or Ray Mk-III or Samus, Meta Knight, Spring Man and Ribbon Girl, those are all "premium" costumes with extra effort put into them.
Ultimately, the design choices don't really make a lot of sense. I don't think there's a pattern to it, they just do what they think looks right for whatever they're going for. Or maybe I can go for the low hanging fruit and say "I sure do remember when Sans put a Gaster Blaster on his arm and started blasting", which is the biggest inconsistency with his costume.
Look back at my first long post about costumes and why they don't have "premium" variants. Geno is not that important. If Sakurai thought in Smash 4 that Geno's costume was a good enough representation of him, then if he has no intention of making a fighter out of him, and has an intention to get the costume back, we're getting that costume. He's not getting upgraded, he never was getting upgraded, there is no "upgrade". I think the most likely scenario, which was dodged for
some reason:
Hero revealed. Comes with his Dragon Quest costumes, and Chocobo. Then we would have gotten a Sans-esque fade to black -> Smash x Super Mario RPG -> Geno's original costume, Beware the Forest's Mushrooms track. He does some vaguely Geno things and that's it. Nothing would have changed, Sakurai would have shrugged and told us it's better than nothing. It still
might happen, rude as that would feel, but I am so confident that we're not going to see an "upgraded" Geno costume I promise you I will buy the thing
for you if he comes with some kind of new head to be more "faithful".