What if... the Oxygen Destroyer was successful?
(The image was to get attention)
So basically I was thinking about this, and what if the Oxygen Destroyer was actually successful in killing both Titans on Isla de Mara?
Well first thing that immediately happens is that any active titans, mainly Mothra and Kong all become the immediately most important monsters. Without both Gs in the picture, there is a real testament on who is alpha now.
Rodan probably gets confused on who's alpha and goes back into his volcano to secretly think to himself he's the King.
Serizawa, Ilene and the rest of Monarch are bewildered. They killed Ghidorah, but at the cost of Godzilla's life too. There's no one like Godzilla to defend the planet effectively anymore. Mark probably gets to bed early tonight smiling.
Alan Jonah also probably gets a little bit worried that his biggest bet is now gone, and decides that they should use the ORCA to awaken the other Titans as a Plan B to set his plan in motion. Emma and Madison are reluctant to do this, and if they tried to escape from Jonah, they probably get killed. Or maybe they only kill Madison as a warning to Emma since (I think) she was the main one using it. They probably kill her afterwards after observing how the ORCA works then.
Monarch is in some deep trouble as they are now dedicated to oversee what happens to the rest of the Titans should Jonah awaken them. By this point, they start putting more emphasis on the new big three as mentioned earlier, particularly Mothra and Kong. Mothra is likely willing to help and convince Kong as shown in GxK, especially since Godzilla is now gone.
Over time, Alan is causing every Titan to be awakened one by one, and clock is ticking for Monarch. Assuming they get make Kong and Mothra get into contact fast somehow, it would help a lot to deal with the Titans. Any longer and the worse the problem becomes. The Kongthra duo manages to have each Titan stop with their ramages, especially after Alan Jonah and Emma (if she's still alive) probably get arrested with the ORCA confiscated/destroyed.
However, the Ghidorah head probably gets purcased by Apex now, which they use to further development of Mechagodzilla. They probably start marketing Mechagodzilla as the solution to stopping these Titan rampages. Though the main issue they still face would be finding a sufficient energy source to charge MechaG with. By this point, Mothra and Kong have developed a very strong relationship with each other, meaning that it would be difficult for Apex to isolate Kong to have him find the Hollow Earth.
In the end, A) Apex use parts of Mechagodzilla to repurpose into anti-Titan defenses to sell to the US military or B) They still find the Hollow Earth energy, though it takes a lot more attempts to do so. In the case for B), the Ghidorah subconsciousness reactivates just like in the original and starts to destroy everything. Walter lives since he doesn't have to do that public monalogue, though his reputation is forever ruined.
With special awareness, Kong and Mothra rush quickly to where ever Mecha's location is at. However, I think this is where they meet their end. Mecha's arsenal is likely to put down Kong quickly, though given the coordination with Mothra as in previous fights the best they could do is slow him down... until Mecha activates the proton scream and fries them both. Plausibly, Mothra could sacrifice herself to a mortally wounded Kong and amp him like with Godzilla in the original KOTM, but since Mothra is so symbiotic with Godzilla and has only spent a few years with Kong in comparison, I doubt it would work. Not to mention Mecha would likely kill Kong before he reaches the supercharged state.
Regardless, Mecha would be challenging other Titans and killing them until he runs out of battery. Monarch reaches Mecha and they decide to make an closed off research zone of his current location. They conclude remants of Ghidorah (plus the skull) were involved with Mecha.
Ultimately, the Monsterverse ends on a pretty bleak note. The Titans we see are either dead, dormant, or causing chaos since there's no alpha. Skar and Shimo don't make it to the surface since Kong is dead. The last footnote in this timeline is a frozen statue of a mimicked King, who was also being controlled by his nemesis.