Other than another alien Titan, the next big bad could be a genetically engineered Titan.

During the meeting at Castle Bravo in KOTM, Dr. Graham states that Alan Jonah has been funding his group of Eco Terrorists by selling Titan DNA on the black market, and if you remember back before the scene where Emma Russel uses the ORCA to wake up Ghidorah in Antarctica, Alan Jonah and his crew steal some samples of his DNA while they are raiding the Monarch Outpost. Alan Jonah could still be in possession of that DNA from Outpost 31 after the events of KOTM. He may even already have some of DNA samples that he extracted from the two Ghidorah heads he had in his possession before he sold them to APEX (according to the GVK novelization).

Also, from what I’ve heard, there is a scene in Godzilla: Dominion where a group of mercenaries capture a Titan named Na Kika in an attempt to take some samples of her DNA before she is saved by Godzilla. Could this group be the Eco Terrorists that we saw in KOTM? If so, this scene implies that they have been attempting to get their hands on Titan DNA since the Mass Awakening.

If Alan Jonah returns in the future, maybe he’ll create a new super Titan using the DNA of Ghidorah and several other Titans. I think a scenario like this is very plausible, and seems like the next logical step for the MonsterVerse to go in. Godzilla has already fought two parasitic Titans (MUTOs), an invasive alien Titan who happens to be an ancient rival of his (Ghidorah), and a robotic doppelgänger that is possessed by one of his rival’s heads (Mechagodzilla). So why not introduce the concept of humanity creating a Bio-Titan (whether it be Biollante or an original creation)?

If we end up seeing a Bio-Titan in the MonsterVerse, it may either show up as a new adversary for Godzilla to fight in another sequel of his, or in a big ensemble film that has Godzilla, Kong, Mothra, Rodan, Behemoth, and all the other Titans teaming up to defeat it. This film would be a reboot of Destroy All Monsters.

TLDR: I believe the next big MonsterVerse villain will be a genetically engineered Titan that’s created by either APEX or the Eco Terrorists. It may either be Biollante or an original creation. The idea of creating a Bio-Titan seems like the next logical step in the MonsterVerse after we’ve had an alien Titan and a robo-Titan.