Ok, speculation time! So, we know Spring Man is the main character in ARMS, and Ribbon Girl is the secondary main character. We know Sakurai usually adds the protagonist of a series first before anyone else from said series. But here's the thing, though. Spring Man is already an Assist, and both he and Ribbon Girl are spirits. Spirits and Assists are known to be "deconfirmations" of a given character, so here's what I think. The character will not only break the rules and ignore the Assist and Spirit "rule", but we'll also get more than one character.
That's right, Fighter's Pass 6 will be multiple characters in one. Spring Man as the default skin, Ribbon Girl as skin 2, Ninjara as skin 3, and Min Min as skin 4. The remaining 4 will be skins of those 4 based on other characters from ARMS, much like Hero's skins. This way, you have the 4 most notable characters from ARMS, a perfect balance of 2 girls and 2 boys, and the 4 of these characters all have almost identical proportions, much like the Heroes, meaning they could easily be the same character. All 4 of them can share moves that may have been exclusive to on of them before, for example all of them could use Min Min's mid-air kicks.
I feel this is the best way to represent ARMS, as it's not just about one character, but the whole cast of interesting and unique characters. To my knowledge, most characters share the same gameplay style but with slight, character-exclusive abilities so it would feel natural enough to include 4 characters in one.