My two cents:
As I've said it before, ARMS is in a weird in that it's not a series, it's a single game that could potentially become a series, but we know nothing about future plans for it. Sprigman is clearly meant to be the game's mascot, being
the basic character used in the cover and most promos, and little doubt his current role as an AT and costume are due to that.
It's also the first series who had some representation beforehand but not a playable character until DLC over a year later. Now we don't know how it's gonna be handled due to what's clearly the main mascot getting in before as an Assist, as they're more than often seen as a death sentence to become playable. The mascot is probably unavailable and there's no defined protagonist.
In my opinion, we're more likely to get someone who is important to the game such as Max Brass or Coyle. Do I believe an Assist can be promoted? Perhaps, I just see it as unlikely after how Waluigi was introduced as an Assist by Sakurai in Smash 4, but it's Nintendo picking, for all we know they could have said "make Springman playable now" or "make playable one of the most popular ARMS characters/a character important to the lore".
Oh well, in the end, I'm the kind of person who is fine with anyone we get, even if there's stuff like me thinking Tom Nook should have made it in before Isabelle, but I won't beat myself over that.
Except fighter pack 11, that's going to another FE swordsman.
-The trailer starts with a red sky, all swordfighters in the roster are running off, covered in bruises and bleeding; the Links and Byleth are trying to contain an angered mob protesting "stop with the anime swordies!" and "wasted slots!" with their bows, alas, they are getting overpowered. Despite that, they managed to take refuge in Garreg Mach Monastery-
-The mob is trying to enter, pitchforks and torches while they hold signs with drawn broken swords and of the Fire Emblem characters being eaten by dogs, all while they shout "No more swordies! Fire Sakurai! No more swordies! Fire Sakurai!"-
-The swordfighters are inside fearing for their lives, Marth, Meta Knight and Ike comfort each other while Shulk and the Mii Swordfighter try to assure Toon Link they'll get out of this, Robin trying to figure a plan to escape... But the door is being hit, they'll soon enter and end their lives...-
-But then, a figure appears, a tired and bleeding Byleth recognises the man and speaks "What are are you doing? You need to get out of here!"-
-The figure smirks at him "I won't be able to do that Professor. It is my job to guard this place, and who would be I to leave my position? And besides..." to everyone's surprise, the man pulled out an envelope with the Smash insignia "Fighters got to keep it together, aren't they?" he grabs a simple nameless sword, and with all the courage he can muster, goes outside-
-The eyes of the new fighter shine, and before throwing himself to the mob, he does a battle cry at the top of his lungs-
"Greetings, Smash community! There's a lot to report!"