You know? After having 2 movies of Godzilla and Kong together, I think it would be time for one of Mothra and Godzilla (I highlight Mothra because I think she should be the one who should have more protagonism). Having Battra, Megaguirus, some of the Kaijus from Rebirth of Mothra saga, or even Gigan (as a reference to Final Wars) as villains. The title being Mothra x Godzilla: The Battle for Earth (If the idea alone is enough, the title guarantees that an army of Godzilla x Mothra shippers will be marching to their nearest theaters. And yes, the title is a reference)