They're not just gonna give Charizard a fireball cuz it'd be useful. Charizard's neutral B is Flamethrower. That's a move he has in his game. What level does Charizard learn fireball at again? Oh that's right, he doesn't have that move, cuz it doesn't even exist.
I do think Flamethrower needs some kind of tweak to make it more worthwhile. Perhaps making it something like IC's blizzard, only with fire? instead of trying to catch people inside it to wrack damage, the way Bowser does, make it into something more useful for spacing people OUT. IC's blizzard can be kind of situational, but there's no denying the utility of it.
Basically, having it be a weaker, quicker, probably smaller, sustainable version of his sideb might work.