I really can't see anyone other than Spring-Man getting in first. At this point I could see Twintelle getting in second. But I wouldn't count on getting a second ARMS character for a while.
Honestly speaking, I really don't think having Spring Man as the default ARMS character is given, unlike many people.
The thing about games with multiple playable characters such as fighting games is that whoever is meant to the protagonist initially and who becomes the face of the franchise and thus the first choice for crossovers isn't one and the same, usually due to the fact that one particular character that wasn't initially important becomes something of a Breakout Character.
An non-fighting game example of this would be Pikachu, who was initially just another Pokemon in the original games, but got more prominence because of how popular he was and thus became the face of Pokemon. Another, more comparable example would be Morrigan, who wasn't originally the main character, but due to her popularity stole the spotlight and became the face of her franchise, and thus is the Darkstalkers character with the most crossover appearances.
Frankly, the only way I could see Spring Man as the absolute shoo-in Arms character is if the roster for a new Smash game was finalised at least pretty early this year seeing as the popularity of ARMS characters couldn't have been measured and they just went for him because he the "main guy". Because frankly, given how overshadowed he is by other characters in terms of popularity, I suspect he will be de-emphasized in terms of marketing in the future and thus no longer the face of Arms.