Calling it now: FP6 is going to be Dr. Coyle. Spring Man's already in as an assist, so unless they promote him to playable, the default character of the roster is out of the running. Dr. Coyle's the last character that was added to ARMS, is the director of ARMS Laboratories and is the biggest bad of the game, with Headlok and Max Brass being the final boss of the arcade mode on lower difficulties, making her the Akuma of the game. She's got the most lore behind her compared to other characters on the roster, and her gimmicks are having a float instead of a jump and having teleporting dodges (like Mewtwo). The mechanics of ARMs wouldn't be too obscured by her relatively simple gimmicks, and she could wield different signature weapons of other fighters like you can in-game.
And if not her? Just give me a Coyle alt color for whoever does get in and I'll be happy.