I really liked the Ivysaur suggestion of a wizard. It got me thinking.
Squirtle will have a ninja costume--> very awesome.
Ivysaur would be a wizard, she could have a wand in her mouth like previously suggested, and her flower-bulb would be reskinned as a wizard hat. The effects could be just visually changed to lightning rather than explosion… IE upsmash becomes a lightning-boom from the top of the wizard hat, and Side-B (razor leaf) would be changed to a more wind-magic looking scythe of sorts, maybe light blue? etc.etc. I think that'd be a really cool skin.
Lastly, if squirtle is a ninja, and ivy is a wizard…
Well then Charizard is a pirate.
He'd have his eye patch, a bandana. A sword strapped to his hip. One leg could be reskinned to a peg-leg, but that would be seen as crippling charizard, so i think it should be avoided. I can't decide what to do with his wings. Do you reskin them to look like pirate flags that have jolly roger (skull+cross bones) on them? That could be cool.
Anyway, i fully support ivysaur-wizard costume.