The nuke that took out the underwater city in Godzilla: King of the Monsters paved the way for more Titans to join the MonsterVerse. Apparently, Ghidorah’s call wasn’t the only event in the movie responsible for the rise of new monsters. In 2019's King of the Monsters, the effects the Oxygen Destroyer had on Gojira forced him to retreat into an ancient, underwater city located somewhere in the Hollow Earth tunnels.
It would seem that this place, which was once populated by people who worshipped Godzilla, is the oldest known civilization in the MonsterVerse. Unfortunately, what was left of Godzilla’s home was destroyed when Serizawa (Ken Watanabe) sacrificed himself by setting off a nuke. The city was lost for good, but Godzilla was rejuvenated by the nuke’s radiation.
Godzilla vs. Kong graphic novel prequel Godzilla vs Dominion reveals the long-term consequences of the explosion. While revisiting the site, Godzilla was beset by a school of giant piranha monsters even he didn’t recognize. Having figured out from their scent that they originated from the Hollow Earth world, Godzilla came to understand these creatures must have come from a breach that was created from the destruction of his lair. In other words, the nuke was so powerful it essentially tore open a doorway to the Hollow Earth. Now, there’s no telling what may emerge from it in the MonsterVerse’s future.
Given the number of Titans awoken by Ghidorah, Monarch already had plenty of giant monsters to worry about following King of the Monsters, but now even more can appear. After all, Ghidorah’s call couldn’t have unleashed every Titan. The monsters that dwell within the Hollow Earth world seen in Godzilla vs Kong likely went undisturbed. But creatures like the Warbats Kong fought, and all sorts of other monsters could detect the breach and make their way to the surface. The fact Godzilla had never seen the piranhas before is indicative of the huge divide between the world above and the world below. There are monsters living down there that may have never gone beyond the Hollow Earth before, but the breach has changed that.
It remains to be seen if the piranhas will be the last Hollow Earth kaijus to stray from their true home. It’s possible larger, much more dangerous threats will follow them through the breach in the years to come. It could be there’s one or more hidden MonsterVerse Titans down there that will use it to invade the surface. This could turn out to be the path that the MonsterVerse takes to introduce some of Godzilla’s kaiju allies from Toho’s movies, or it could be how Godzilla 3’s villain enters the picture.