I, personally, am predicting Spring Man because something to note: We've never gotten a series playable debut without the mascot/protag. I believe I mentioned this some time ago. It is certainly a rule that could be broken, but Smash is a mascot fighter. That would be a strange sudden shift just because a character is already an AT. The fact that ARMS is a "special request" seems to suggest something like that as well.
The context of the "special request" phrasing could be a variety of things.
The mere fact that they were tasked to implement a character from ARMS is a "special request". It could also be to infer the mechanics incorporated to the fighter were requested too,
since the existence of them stands out from the translation of his statement.
I think without any context, it's not very wise to infer a suspicion like that on such an open phrase. As far as we are aware, the context that a new character could already be a game element may not be a huge deal to Sakurai and his team.
I also believe it is worth noting that ARMS, unlike the majority of other fighters selected in the past from "new titles", is in a rather unique position. Fighters that are usually looked as "promotional" in nature, are usually decided when that game is in development. With that in mind, the character selected is usually the one designed the earliest, and whoever they choose to use in promotional material. This is not only because it assists the brand recognition, but it's because they have no data and what characters are popular with the playerbase (as they have no playerbase at that time).
ARMS has been out for two years at the time of this decision to add a character, and they have plenty of resources to measure which characters are used and popular through the game's online (as they've gone on to say that
Ninjara was the most popular character used online a couple months after launch) as well as the party crashes to see which characters have the player support as well (Granted, this is also based on wins / loses but it is still something worth considering).
So unlike before, the team knows what characters are popular. I don't think this information would be overlooked if the ARMS team was approached about selecting which fighter should be added.
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And I mean, sure, they could still just say "add the cover man obviously" but i feel like given that we know what characters have been publicly commented on by the game's director Kosuke Yabuki there is a lot of reason to raise a brow to this character just being a simple "mascot" addition here.