So I wanted to bring up Mii costumes again.
Top row: Skull Kid (Possible Playable Character), Megaman X (Final Smash Asset), Protoman (Final Smash Asset), Dunban (Final Smash Asset), Isabelle (Playable Character), Heihachi (Possible Playable Character)
Second Row: Akira/Jacky (Possible AT's/Unconfirmed), Zero (AT), Megaman Exe (Final Smash Asset), Inkling (Playable Character), K. Rool (Playable Character), Flying Man (Stage Asset)
Third Row: Chrom (Playable Character), Lloyd (Possible Playable Character), KK Slider (Possible Echo for Isabelle now/Unconfirmed), Toad (Peach asset), Viridi (Stage Asset), Hunter (Unconfirmed/Rathalos/Possible Playable Character), Chocobo (Unconfirmed/Possible AT)
Final Row: Geno (Possible Playable Character), Takamaru (AT), Ashley (AT), Gil (Unconfirmed, Possible AT or Playable Character), Tails (Unconfirmed, Possible Echo or Semi-clone, Possible AT), Knuckles (AT)
As you can see, the majority of the costumes are SOMETHING in Ultimate. It's very likely that nearly all of these have some form of presence in the game. Of these: Skull Kid, Heihachi, Lloyd, Slider, Hunter, Geno, Gil and Tails are possible playable characters. Any of these can be assist trophies as well, in addition to Chocobo, however Skull Kid, Heihachi, Lloyd, Tails, and Geno are considered probably the most popular of these selections, as with Isabelle, to become fighters. What we DO know is Square is getting a second rep from Vergeben and it could be Geno, but we don't know. Additionally, we don't know if Bamco will follow this trend of 3rd party companies getting additional characters (so far they only have pac-man), but if they were and judging from these costumes, it'd be either Heihachi or Lloyd.
If I had to guess that Sakurai was still pulling from this selection, considering we got Isabelle, Inkling, K. Rool and Chrom from it already, I'd say:
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Skull Kid: First Party Villain (Zelda hasn't had a proper newcomer since melee), the only remaining first party character on this list with a strong backing.
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Heihachi/Lloyd/Gil: Bamco 3rd party (They are making the game with Sakurai and made 4. Every other 3rd party company has 2 or more characters now (counting square if Vergeben is right). Honestly any of the 3, but probably only one.
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Geno: Possible SE 3rd Party (Was a standalone Mii costume update and the only one with a title card. Sakurai loves this character. I'm going to guess the SE character is him)
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Tails: Yes, I know Shadow is more likely, but with Isabelle being revealed, I can see a Luigi-fied semi-clone as Tales for Sonic anyhow. I could even see him using his blaster. We'll see.
Tl;dr: We know many of these costumes, according to Sakurai in previous interviews, were specifically made as stand-ins for requested characters. They are specific acknowledgments to being popular, for the most part. Pay attention to them. 4 characters out of the 9 new confirmed COME from these Mii costumes.