After some reasoning and research, I think I have an opinion on who is more likely as an ARMS rep.
I found 4 characters, who are (not in order) Max Brass, Mechanica, Dr Coyle and Misango.
I still don't consider characters who are spirits in the game as likely until the "rule" is officially deconfirmed.
-Max Brass seems to fit the champion/wrestler role similar to Incineroar, and he's designed around the theme of fighting (no ethnic or random secondary themes such as clown or police and stuff). So, he works well as "generic" fighter to introduce people to the franchise.
-Mechanica would be unlikely if not that there are some people who compare her to some Overwatch characters (mainly D.va, but she also wears glasses that resemble Tracer's). They may decide to play around that with implicit references to increase her popularity and create viral marketing, similar to how the new Doom game was unofficially promoted with Doom x Isabelle/Animal Crossing memes, even if the two games have nothing in common aside of the release date.
-Dr Coyle is apparently the villain of the game or something like that... being slighty more important in the story of the game, she may have a better chance than others... though it's also possible that she's the legendary spirit instead.
-Misango seems to have a spirit companion gimmick that, from what I've read (I didn't play the game) sounds kinda similar to Shulk's Monado arts. He's also pretty basic concept-wise, aside from the ethnic theme, he's just the regular human figher with regular stretching arms.
I'm still not sure who's more likely among those four, but I'm leaning more toward Misango... there are several reasons, one is that most of the DLC characters are usually gimmicky, and the spirit companion thing would fit that trend. Also, I was joking in my previous post, but not even that much; Sakurai is a Street Fighter fan, and long ago I did a post trying to predict what already represented franchise would get another character: I speculated that Street Fighter was one of the most likely. ARMS is not Street Fighter, but if I'm even just partially right about my prediction, the ARMS character may turn into having some moves inspired by Dhalsim like Incineroar takes some moves from Zangief. Both Misango and Dhalsim have an ethnical theme, even though they're based off different cultures... it would help to make the character feel similar, like Incineroar and Zangief are both themed around wrestling. Both characters have stretching arms as main gimmick.
There is a very small possibility that the character is Spring Man and the assist trophy gets replaced with Springtron who looks very similar/share the same body proportions. I think this is not happening, because some people will not buy the DLC and I don't see Nintendo removing the mascot of ARMS as assist trophy from their game only to replace him with a more obscure robot clone. If it was a free update for everyone maybe.