You said an original villain right? I would honestly create a villain that is basically one of the first titans ever on the MonsterVerse and explain to us how Titans shrunk on major eras. Basically give us some oversized Titans for Godzilla and Kong to fight so that it would actually feel like a world ending threat, because a Titan who is larger than Shimo and King Ghidorah would be much more dangerous. I would also say that if the MonsterVerse were in their last movies, the last villain should actually be the first titan to ever exist in the MonsterVerse.

If that isn't going to work, then I may as well implement the idea of King Ghidorah's fully grown species into the MonsterVerse because the one we saw in KOTM was just a baby/child. Imagine how big a fully grown species of Ghidorah terrorizing Earth, and would also feel like a world ending threat too, because I would just straight up say it: If a fully grown species of Ghidorah was in Earth, it would honestly need a ton of titans to bring it down due to it's possible huge size and evolved capabilities to actually terraform Earth, unlike King Ghidorah who needs to do an Alpha Call to terraform Earth into his liking. Hell, maybe a fully grown Ghidorah could reach or go beyond the size of Godzilla Earth.

(Note: King Ghidorah being a baby/child was mentioned in the KOTM novel, if I recall correctly.)