15. Byte & Barq
As much as I love these two (they were my go-to secondary fighter!), I think they are the least likely choice, for one main reason: Barq. The stretchy arms are gimmicky enough as-is, adding in a duo-fighter mechanic would simply overcomplicate things.
14. Misango
He's cool, but he doesn't have the popularity or "star power" to stand out as the one to logically represent ARMS as a whole. Plus, his little spirit buddy poses a similar problem to Barq.
13. Springtron
He's a clone of Spring Man. Why include him over Spring Man? It would feel really weird to have Springtron playable and Spring Man as an assist trophy.
12. Lola Pop
Similar to Misango, I don't think she has the recognizability or the "star power" to be the one ARMS representative in Smash.
11. Kid Cobra
Basically the same as Misango and Lola... he doesn't "spark joy" as a contender for Smash.
10. Mechanica
Since she's just a regular girl with normal arms, her design doesn't represent ARMS as well as other characters. She's a bit higher on the list because she's one of the more recognizable ARMS characters, but I don't see her getting in.
9. Twintelle
Yeah, I know she's one of the most popular ARMS characters, but she has the same issue as Mechanica - she doesn't represent the game well, because she has normal arms! ARMS is about people with stretchy arms, so I think they're gonna add a character who has them.
8. Master Mummy
He's one of the more recognizable characters, however, I just don't see him being "the guy" to represent ARMS as a whole. The only reason I'd see Sakurai including him is for balance reasons - having the character be slow & heavy to counterbalance the crazy range.
7. Ninjara
Another fairly recognizable and well-liked character. He's basic enough in terms of design that he could feasibly represent the game as a whole. But at the same time, his design doesn't bring a whole lot of uniqueness, since we have ninjas on the roster already.
6. Spring Man
Yes, he's already an assist trophy, but who says its a hard rule that a character can't be playable and an assist? He's the poster boy of the series so it makes the most sense. However, if they were including him, I think they would have just announced him outright.
5. Helix
He's a popular, recognizable character with the perfect level of weirdness to stand out in Smash! He perfectly represents the quirkiness of ARMS, and I can't think of a better character to convey the theme of stretchiness. I think he has a decent shot of getting in.
4. Dr. Coyle
I could honestly see Dr. Coyle being included. She has a unique design, she's one of the more popular characters, and she has significance to the lore of the game. Plus, she would be unique as a female villain - I could see her piquing Sakurai's interest.
3. Max Brass
He has more than enough significance to the world of ARMS to warrant inclusion in Smash, being the final boss of the Grand Prix. Plus, he's pretty well-liked. And like I said for Master Mummy, being a heavier, slower character might be good for balance reasons.
2. Ribbon Girl
I think she's a very likely candidate given that she is basically the secondary mascot character after Spring Man. She's basic enough that she could function as a representative of the game as a whole, but is still very unique design-wise.
1. Min Min
She's one of the most popular ARMS characters, she has a unique design that would stand out in Smash, and a simple but unique mechanic (her dragon arm) that would enhance the stretchy arms gimmick rather than overcomplicate it. Plus, Min Min was the ultimate winner of the Party Crash Bash - Nintendo has used these sorts of things to guide future content, such as the Callie vs Marie splatfest, so I wouldn't put it past them to use the Party Crash results to determine who gets in Smash!
Of course, another option would be to have a multiple-characters-in-one situation by using alternate costumes - I'd go for Spring Man, Ribbon Girl, Ninjara, and Min Min, with one alternate palette each, a-la Hero. But if I had to bet, I'd put my money on Min Min!