I had this massive fan theory last night on how the 
Legendary Series may end. Keep in mind it was midnight here, so my English may not be very good because I was very tired and these were tired thoughts...

So Legendary introduced a new enemy like the MUTO, right? And the second movie is going to have Ghiddorah, Rodan, and Mothra? Well, every trilogy has to have a definite end. I thought that end is likely going to be Godzilla's death. So maybe a few years after Godzilla fought Ghiddorah with Rodan and Mothra (possibly killing all three), what if Mankind had been abusing the world's resources, knowing that there are so many hidden things like giant monsters, so they look for more natural resources under the ocean? Mankind then exploits more natural resources -- using up a lot of fossil fuel to be able to find these new resources, causing mass pollution of the world. This then makes Godzilla's enemy Mankind itself.

My idea is that Mankind is tipping the balance of nature by destroying nature -- polluting the planet. Godzilla must restore balance somehow, so he destroys nuclear energy sources, and cities by himself (thus paying homage to the original Godzilla with a new theme of environmental warning rather than nuclear holocaust) and in the end, mankind is so desperate to save itself creates a weapon to destroy Godzilla: the Oxygen Destroyer, removing any oxygen needed to sustain Godzilla while keeping him distracted at a high enough sea-level that he doesn't get any radiation from the Earth's core.

(Now I got excited enough that I even thought up an ending to the movie) The movie ends with either with Godzilla disintegrating like the old one, or his body slowly floating down to the ocean depths, where we slowly fade to a shot of a lone egg, next to his body. Suddenly, egg cracks and a streak of blue fire comes toward the screen, and we hear the signature roar one last time. I know it sounds super environmentalist, but it makes total sense...