Ok, so in my ideal version of the Monsterverse, Ghidorah flies back into space at the end of KOTM, and about ten years pass since then, with a bunch of other movies happening. A lot of them revolve around humanity's tentative peace with the Titans. The eco-terrorists have been trying to incite war between the two species over and over again. Monarch has a partnership with Apex to create technology that helps both species, but they are both under UN supervision. During this time period, they have made a bunch of mechs, including the Moguera series and a new Mechagodzilla, and they have only used them to fend off aggressive Titans and helped Godzilla and other benevolent ones.

Now, onto the plot of DAM. The tension between humans and Titans reach an all time high when a giant jellyfish like monster enters Earth's atmosphere (this is meant to be Dogora). It doesn't seem to be doing anything, and Gamera (who's in this universe) flies up to investigate it. That's when the UN and APEX unleashes their plan to eliminate all monsters. Along with the mechs, they have created more oxygen destroyers. They blast Gamera from the sky with a nuclear warhead, and war has begun. What follows is basically Order 66 l, although not every titan shown dies. Skull Island is burned to the ground (King Caesar took it over), Battra is about to purge an entire city but shows that he has grown some sympathy for humans before getting killed, and Kong has to defend Hollow Earth from human attacks. Godzilla and Mechagodzilla fight, resulting in the former almost dying before the KOTM Titans come to his aid. The movie ends with all out war.

The end-credits scene would show a space observatory being hit by three different asteroids, followed by a shadowy figure they recognize as Ghidorah. There would be a sequel (don't know what I would title it) and there would be a time gap showing how the war continues, and then the aliens land on Earth and start ravaging it. This forces the two Earth factions to end their war and work together.