Typically I see people saying Destoroyah should be the Monsterverse's final opponent, if not something original, or even Bagan.
But what if, I'm just spitballing here, we take one of the absolute LAST Kaiju anyone is expecting and make them the Final Boss?
That's why I've been ride-or-die for the idea of Dogora as an ultimate villain. Unless you REALLY know your Toho, you probably don't know who he is, buuuuut he's such a unique, singular entity. There hasn't been a Kaiju like him before or since. He's so malleable that he could either work as a grim Titan image of entropy, or the coldest, cruellest villain to ever set tentacle on a cosmic scale. He could be the truest representation of The End of Days itself that humans and Titans alike must cooperate to stop.
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Though if I were to go into my own ideas on kaiju baddies for Godzilla media, we'd be here all day.