But that's just the thing: it's not your decision who is and isn't a playable character in the game. It's Sakurai's. Simply because you don't think they would work as a character doesn't mean they won't actually be a character.
This goes back to the whole "don't dismiss a leak simply because you can't comprehend it" issue. That's more of a character discussion topic. If you think Sal's leak is invalid for reasons like he got his information incorrect or you know the leaker is lying, that's different, but disputing it because you don't think a particular character could possibly be playable really doesn't hold much ground. Any character, given direction under Sakurai's hand, could work. He could turn the Virtual Boy into a character if he wanted, or the GameBoy Camera, as @
CyberWolfBia
stated, but whether you agree with it or not doesn't make it any less valid.
Sal's leak may be false, but many sources deem otherwise. If there were something to disprove the leak, it would be dead already, but since it's the only leak that seems to be holding its ground, it stands true that something about it must be pointing in the right direction, whether the characters listed are popular or not.