The main mystery of the trailer for Godzilla vs. Kong was the revelation that the King of the Monsters was acting wildly out of character by directly attacking humans. Several theories have been put forward of what might be causing his turn to villainy, but the simplest is that it’s not actually Godzilla responsible for the destruction, but the impostor Mechagodzilla.
Mechagodzilla, as you’ve likely already surmised, is a robotic duplicate of the Titan who is, along with King Ghidorah, generally considered to be his nemesis. It first appeared in 1974’s Godzilla vs. Mechagodzilla, where it was depicted as a creation of an alien race that planned for it to take out Godzilla to pave their way for world domination. It had a covering of scales imitating Godzilla, only for the ruse to be revealed when it was attacked by the Titan’s ally Anguirus, and a portion of the disguise was torn off to show the metal beneath.
It’s a distinct possibility that this precedent of Mechagodzilla initially masquerading as Godzilla has been repurposed for the new movie. After all, the robot’s presence has already been spoiled, and brief allusions from the trailer seemingly confirm its malevolent intentions.
If Mechagodzilla is the one running amok, the plan is probably to make humanity turn against Godzilla and destroy him, thus preventing him from stopping whatever nefarious follow-up scheme has been put in place. Kong may come into it from believing he’s attacking the real version (and at one point may well encounter him and carry on the fight, unaware of the lizard lord’s innocence), until the true villain is revealed and the ancestral rivals end up uniting against the common enemy.
There will probably be at least one more trailer for Godzilla vs. Kong before the film’s release in March that may offer more tacit references to the metal menace, but the main reveal will likely be left for the movie itself. As it should be.