So, I know the vague indication in KOTM is that Ghidorah came from outer space, there's a line where it's described that he fell from heaven. But it occurs to me that the writers deliberately left that wide open to interpretation, and that Ghidorah might not be alien in origin at all, or only partially.

The reason I say that is, based on who saw it happen, Kong fell from "heaven" also, but he's definitely not alien. The ruins that Godzilla lived in, the Iwi who went to Skull Island with Kong's race when they were exiled, they all came from Hollow Earth. Ghidorah may have been alien in the context of he wasn't a Titan from Hollow Earth, but a Titan who evolved on the surface of the Earth. If Ghidorah was after the power in Hollow Earth, and found a way in the same way Kong did, in the moment of inversion when gravity changed he would have plummeted briefly before regaining his bearings. And everyone living down there would have seen this terrifying new entity coming into their world, seemingly from heaven itself.

It would also explain how he ended up frozen in the Arctic, especially since he seems to shun colder climates and wouldn't go to them voluntarily. Godzilla could have dragged him down any number of tunnels in an attempt to drown him, and Ghidorah wound up getting swept into an arctic current and becoming imprisoned. Godzilla might not have finished him off simply because he lost track of him when Ghidorah froze and was basically shut down.

Just some fun food for thought, a way to nod to Ghidorah's original origin without explicitly tying this version to it. .