8. Kong Heard Ghidorah's Call
After 2014's Godzilla and 2017's Kong: Skull Island, King of Monsters served as the third film in Legendary Studios' MonsterVerse. The franchise's first 'Avengers moment' is set to come in 2020 with Godzilla Vs. Kong which, naturally requires a lot of set-up, character and world building. Not all of this comes within the movies themselves, however.
Aside from its 23 movies, the MCU canon expands to TV shows, comic book preludes, and several one shot DVD bonus features. In the same vein, the official novelization of Godzilla: King of Monsters offers some insight that never made it to the movie itself.
The most important of which, in terms of building towards next year's crossover, is King Kong's appearance. In a chapter told from the beast's perspective on Skull Island, he actually hears the call from Ghidorah to wake the other titans. Having no interest in leaving his home however, Kong decides to ignore Godzilla's adversary.
Taking this into consideration moving forward, it must be then that some kind of trouble, or perhaps even Godzilla himself, will disturb the inhabitant on Skull Island to give him a reason to enter the fight.