My thoughts on every ARMS characters' possible inclusion:
Spring Man
His inclusion as an Assist Trophy gives an opportunity to break a fan rule, which would make for a fun and even a little surprising trailer. However, his role as the milquetoast main character would equally take away from the excitement. Middle of the road in terms of likeliness.
Ribbon Girl
Her innate abilities, multiple jumps and fast fall, fit Smash like a glove. However, other female ARMS characters outshine her in popularity. Like Spring Man, middle of the road in terms of likeliness.
Ninjara
Placed 2nd in the final Party Crash, so the popularity's there. However, can Smash afford to add a third ninja before a pirate? Also, Ninjara is very elusive and wants to often get close, which is kinda at odds with the uniqueness an ARMS character would bring to Smash as a long-ranged melee fighter. A little above Spring Man and Ribbon Girl in likeliness.
Master Mummy
Could bring a very unique playstyle as a long-ranged heavy who doesn't have to approach because he can heal at will. Not among the most popular characters however, so not very likely. Perhaps the unlikeliest.
Min Min
Winner of the final Party Crash, basically making her the most popular ARMS character. Her trademark kicks and projectile spewing Dragon Arm would naturally allow her a more varied Smash moveset than many other ARMS fighters. The most likely pick in my opinion.
Mechanica
Relies on her mech suit to be able to compete with others in ARMS instead of the natural ability, which would make her inclusion feel... wrong somehow? Not very likely.
Twintelle
Among the most popular characters. Would also tick a very important box for some, being the first black female fighter in Smash (discounting alts) if included. Also uses her hair to fight, which would allow for some very unique animations. Just behind Min Min in likeliness.
Byte & Barq
Their role as a pair character would step on the toes of Ice Climbers a little. In ARMS, Barq is also very much bound to the ground, which could make her inclusion in an air-heavy game like Smash somewhat problematic. Not very popular either, and thus not very likely.
Kid Cobra
A cool character that could bring his own unique flair if he'd take his snakeboard to Smash as a special move or something. Being among the more popular male characters also increases his chances. Maybe around Spring Man in likeliness?
Helix
Would be a cool pick. His weird and funny special abilities would feel right at home in a wacky party game like Smash, but I could also see them being difficult to implement. Not a very likely pick though.
Max Brass
Would bring with him many of the same tricks as Spring Man, but his boastful and macho personality give him more appeal. If we assume each fighter would be paired with their own stage, I feel Max's would make the most sense in Smash. Competes with Ninjara for the spot of the most likely male character.
Lola Pop
She is the most likable clown ever, but I don't know if that has anything to do with her chances. One of her gimmicks is being able to move while shielding, which would be very interesting to see in Smash. Maybe she could have some original clown-inspired moves in Smash like juggling balls. I want to say she's the likeliest of the not very likely.
Misango
I feel that Misango would step on the toes of Shulk as a character that can change his attributes at will. His ARMS are the only ones with the damage-over-time poison element, which is unique in his own game, but not as much in Smash if you count things like Lip's Stick, Eiha and some Assists. Not very likely.
Springtron
Even though he has his own EMP gimmick, I think he'll only get in as a Spring Man skin or Echo. Thus, like Spring Man, he's middle of the road in terms of likeliness.
Dr. Coyle
My absolute number 1 pick. Would tick off the coveted box of
unique female villain. Made a moveset for her and everything. Sadly I don't see her as a very likely pick as she isn't among ARMS' most recognizable faces, being released as the last DLC character when the general public had already moved on from the game.
Her theme better make it in however or so help me...
