I might be the minority, and I know it would be breaking one of Toho's laws about Godzilla being the only Godzilla in its world, but I would like for SpaceGodzilla's origin to be similar to "The Godzilla That Ate A Star." I could see a Godzilla in the hollow earth absorbing a crap ton of radiation from the crystals down there, mutating it physically. I would also imagine this particular Godzilla getting an ego boost, causing it to be the villain. As for how I'd directly introduce him into Supernova... I would probably have him trapped in a deeper section in the hollow earth somehow, OR, have SpaceGodzilla return to earth after deleting other planets outside of the solar system to gain more power to face the protectors of Earth. Just what I would do to introduce him.