Between Min Min proving that base game roles can be reimagined and Bomberman showing that a character can be simultaneously playable and an AT, there's really no reason to assume that Isaac couldn't be playable if that's what Sakurai and Nintendo choose. That argument is dead and buried at this point, although everyone is entitled to their opinion.
That being said, with only four spots left, nobody is "likely" at this point. Min Min and Bomberman simply prove that more options are on the table than there were for fighter's pass one, which still includes, y'know, the entirety of gaming.