I'mma just get my ratings of likelihood out.
Characters who have a high chance:
Helix: Possibly the most familiar face outside of the mascots. Helix's movement and general demeanour are very thematically fitting to Smash, he has great mobility options and a good stage to boot. He also plays major roles in side modes.
Max Brass: The main villain is ARMS, so anyone who's played will know him, and he's the driving force behind the tournament as a whole. He may also be able to switch between a light and heavy movement style, giving him advantage over the ARMS heavyweights. It also makes sense to have a main villain, since the main hero is an Assist.
Dr. Coyle: Shares some of her reasons with Max Brass. She's a very important character to ARMS as a whole, inventing Helix, Lokjaw and Springtron, her moveset could incorporate other character's arms without feeling out of character, since she gave herself the Arms ability, and if we take Bayonetta's texture changing glitch into consideration, Coyle is the ONLY character who changes texture in battle due to her invisibility. I'd give her front running. honestly.
Characters with a medium chance:
Kid Cobra: He's very vanilla as far as Arms goes, but his mobility gives him an edge where Smash is concerned. His stage would also be interesting.
Lola Pop: She's got the feeling of Smash down and a plausible blocking move, but she's not too unique otherwise.
Characters with a low chance:
Master Mummy: Having a heavyweight represent ARMS as a whole wouldn't be the best choice.
Mechanica: Same reason as Master Mummy.
Byte & Barq: They're a gimmicky character in their own game, so they'd have two gimmicks on top of each other which may break balancing.
Misango: Honestly, most people don't even remember he exists.
Springtron: Would've maybe been a costume/Echo for Spring Man, but without Spring Man, he's not got much going for him.
ARMS 2 Character: It's an interesting concept, but I don't think they'd show the ARMS 1 roster only to announce a sequel.
Characters with no chance:
Spring Man: He's an Assist Trophy and would've already have been revealed if it were him.
Ribbon Girl: She's a Spirit.
Ninjara: He's a Spirit.
Min Min: She's a Spirit, and it's likely the Party Crash Bash didn't actually mean much outside of ARMS itself.
Twintelle: She's a Spirit and doesn't fight with her arms, misrepresenting the series.
Biff: He's just the announcer, not much here to work with.
Lokjaw: He's better suited to a cameo in a Final Smash.
Alternate Costume Characters: [laughs] Come on, if you know anything about ARMS, you know how different the characters' body types and abilities are, and the only few that could much this concept are already Spirits. I know this theory only happened because they showed the whole roster, but guaranteed, it's not happening.