Although Jack Jack's powers aren't readily apparent, it should be noted he rotated among three different forms, but each seemed elemental in nature, rotating through a being of all fire, a being of some sort of very heavy metal, and a devil-looking form. Although this could be a Multiform, at this point it's probably best to represent this via a VPP, with a limitation being that the VPP power(s) in play at any time must all reflect a simple elemental type of being.
Combine this with some dice of "Fire Form" for the full devil-being effect.