My way of seeing it is this.
I'm almost 100% sure it'll be Spring Man. Like it or not, he gets main character status among the ARMS cast. He's first on the roster. He's the Assist Trophy. His spirit is the first ARMS spirit numerically. He's the one with a villainous copy in Springtron.
The only way I see it going any other way is if the dev team specifically requests someone else, likely Min Min considering they also really like her. But since Nintendo is deciding the line up anyway, I can't realistically see anyone BUT Spring Man.
My go to example is them picking Byleth over Edelgard. Everything people use to justify a different ARMS character (let's say Min Min for consistency) over Spring Man and more could apply to picking Edelgard over Byleth.
Edelgard was, and still is even after Smash, significantly more popular than Byleth. In fact, Dimitri and Claude are too. All three House Leaders are. Min Min is far more popular than Spring Man.
Edelgard is inherently more interesting than Byleth, as Byleth is intentionally a blank silent slate and Sakurai said personality is important. Min Min has a neat backstory involving her family's restaurant while Spring Man is your generic energetic protagonist.
Edelgard is more unique due to a starkly different weapon type and gameplay style, being a heavyweight axe user. Byleth is another sword user, albeit the Sword of the Creator is a whipsword (which ironically enough got UNDERPLAYED in Smash). Min Min has a dragon arm and kicks while Spring Man is basic.
Edelgard was all over the marketing and promotion, just like Min Min after her popularity became evident.
And they still went with Byleth at the end of the day. Not Edelgard. I'd even go as far as saying Edelgard had a better potential chance than any non-Spring Man ARMS character because she's a major player in the game, being at worst the third most important character on the route that focuses on her the least, and literally having the main theme of Three Houses, Edge of Dawn, be about her.
If Edelgard didn't get picked over Byleth despite having a substantially good résumé, what chance does anyone BUT Spring Man have?