Okay, this is a long read, but bear with me here since this is just one of the ideas I've had regarding rewrites of the Monsterverse:

First off, I would keep the plotline used in the end credits of KOTM, where the now awoken Titans are heading straight for Skull Island after Godzilla essentially banished them from the mainland. At the same time, the alpha call Ghidorah produced back in 2019 has awoken hordes of Skullcrawlers and various other creatures, possibly even a Warbat or two, but Kong has fought them all, leaving Skull Island a wasteland of decaying bodies and skeletons. I would also add the fact that Camazotz's attack has caused the storm system around the island to move inward and become more turbulent, but Monarch managed to build a rather vast containment site on the island, including an entry-point into the Hollow Earth (similar to the prologue in the GvK novelization). Keep Team Kong, but instead, have Ling Chen, an older Lin San, and Houston Brooks be the head researchers at the outpost with Jia being in their care. With the Titans running rampant, Monarch has a plan to use Godzilla's bioacoustics to lure the Titans into the Hollow Earth before blowing up every entrance, but the plan fails and Nathan's brother, who is also Ilene Andrews' fiance in my story, dies in the Hollow Earth, leading to Ilene hating Nathan because it was his plan. Change Team Godzilla entirely and replace them with the characters from KOTM, with Mark and Chen, who in my rewrite are a couple, as the leads. Madison Russell is still very prominent as she is in the actual film, but ever since KOTM, has had nightmares and PTSD-related flashbacks revolving around Ghidorah, to which she has a way of connecting with Chen through dialogue since Chen also has visions of various Titans, especially Godzilla and Mothra. Make Ren Serizawa the CEO of Apex because he wants to destroy his father's legacy by killing Godzilla and the other Titans and be seen as humanity's hero, and he will go to extreme lengths to achieve that goal; I would also add a cameo of Charles Dance and his team from KOTM as the ones who brought Ren the Ghidorah skull. After they give him the skull, Ren kills Charles and his group before building a brand new Orca with Ghidorah's call to lure Godzilla across the globe, making him attack various cities and Titans to make him seem like the villain, including attacking an Apex facility in Pensacola in a blind rage, not only destroying the complex but the entire city as a whole. Ren goes to Team Kong and discusses the details of the mission, bringing along Team Godzilla, where we have a big and happy reunion.

Now, the whole "hollow earth energy powers Mecha-G" plotline just feels completely unnecessary, so instead, have them want to explore the Hollow Earth to find the perfect location to lure the Titans, including Godzilla, so they can seal them down there forever. Instead, we would have Apex using Hong Kong's power grid to temporarily power Mecha-G. Also, in my version, the H.E is not the center of the planet, but now, massive, cavernous pockets in the Earth are lit by enormous bioluminescent trees while others are only lit by volcanos, bioluminescent animals, etc; I would even love different biomes such as underground deserts, tundras, rainforests, and even underground oceans. While Team Kong and Team Godzilla aren't looking, Ren plants Apex spies into outpost 33's personnel who hack the satellites to blare Ghidorah's calls to lure an enraged Godzilla to the island while bringing their own seafaring vessels to go back and forth between Skull Island and the mainland, including the cargo ship we see in the film and the aircraft carrier. After a small fight between Kong, an alpha Skullcrawler, and a juvenile Warbat, Godzilla appears on the radar, swimming past the storm system and towards the shallow coasts of Skull Island. Now, the ocean battle/Round 1 happens in the shallow area of the island with Kong and Godzilla standing on a beached aircraft carrier before Godzilla knocks Kong backward into the island, allowing us to get into a fight on Skull Island; Round 2 starts, and Godzilla battles Kong, destroying most of the island in the process and inadvertently destroying the iwi village, killing all except Jia. After destroying the Monarch site, Godzilla is about to kill Kong when Team Godzilla discovers the Orca and breaks it, causing Godzilla to snap out of his rage before going back into the sea, leaving Kong unconscious and the most of the island in ruins. Team Kong and Team Godzilla go their separate ways after a bad argument about Godzilla's behavior, and Team Kong uses Apex's HEAVs to go into the Hollow Earth while Team Godzilla uses the Argo to follow behind Godzilla.

In the Hollow Earth, instead of mere missiles, the HEAVs have electric harpoons called "Shock Anchors" and "G-Crushers", the same sort of weapons found on Heisei's Mechagodzilla. Another weapon they have which will come in later is a larger maser cannon, capable of shooting a beam of concentrated electricity. Remember that, because that will be useful later. After briefly fighting a giant dinosaur, who is an homage to Gorosaurus, Kong makes his way to the temple to see a graveyard of Kong and Godzilla skeletons. When they enter the temple, they find weapons, toys for the baby Kongs, and cave paintings showing the ancient war; Houston comments on how the myths were real, there was a Titan war, and the current Godzilla and Kong are the last ones standing. Some of the cave paintings are of the Godzillas killing and eating infant Kongs, a tribe of Kongs holding a severed Godzilla head, and of a war between the Godzillas, Kongs, and other Titans against a winged crystalline Godzilla known as Zo-zla-Halawa. Jia and Ling help to tell the story too, with Andrews and Houston translating it for the others. Meanwhile, Godzilla heads straight to Hong Kong, attracted by Ghidorah's call produced by the Orca built by Ren. Mark is keeping in contact with the Hollow Earth mission while Madison and Chen try to figure out Godzilla's erratic behavior and why he's heading straight for Hong Kong, slowly suspecting that Apex and specifically Ren have something to do with it. After the test with the Skullcrawler we see in the film, Ren is ready to activate Mechagodzilla in order to kill Godzilla, but then, after destroying a few blocks with his tail and atomic breath, fires his atomic breath down towards the Earth, opening a hole to Hollow Earth. Ren realizes Godzilla knows Kong is down there, but he decides to kill Godzilla no matter what is in his way and unleashes a swarm of Skullcrawlers from the Apex facility beneath the mountain. After a brief fight between Godzilla and the Skullcrawlers, Kong leaves the Hollow Earth with his axe and climbs up to Hong Kong, starting Round 3 as the sun sets into the night.

My version of the fight would be very different and much more brutal, as Godzilla and Kong savagely fight throughout the cityscape. We have a moment where Team Kong realized that Apex plans on killing all the Titans, including Godzilla and Kong; initially, Andrews and Nathan want to shoot Godzilla with the HEAVs' G-Crushers and Shock Anchors to help Kong, but Houston, Ling, and Lin San think that Godzilla will attack them if they do, so they meet up with Team Godzilla at the Monarch outpost. With evidence proving that Ren had something to do with Godzilla's recent attacks, both teams head to the Apex headquarters in Hong Kong to confront Ren, who is about to activate Mechagodzilla with the Ghidorah skull. After Godzilla tears a chunk off of Kong's shoulder and Kong slashes Godzilla's chest open with the axe, Godzilla is about to kill a group of humans below, but Kong rushes in and the "roar-off" happens but Kong loses, and Godzilla knocks him out but doesn't kill him. Inside the Apex mountain, Team Godzilla and Team Kong confront Ren, who points a gun at them, and goes on a speech about how his father abandoned him all his life for Godzilla, even dying for him, and now he'll take away the one thing that stole his father. After a brief struggle, shootout, and chase scene, Ren finally activates Mechagodzilla and "drifts" with Ghidorah's skull similar to that of Pacific Rim, to which we get a psychedelic scene where Ren sees a vision of Ghidorah before the three-headed monster essentially swallows his mind, but not enough to completely overtake him. Both teams realize what's happening and order Monarch to attack Mechagodzilla with both G-Team and the Chinese army, while they try to find a way to disable Mecha-G; Ghidorah's spirit, however, refuses to stay dead, and while Ren attacks Godzilla with Mechagodzilla, the sudden drifting with its consciousness reactivated it.

Ghidorah takes over and Ren's mind is mixed together into an entirely new, malignant consciousness, allowing Ren to have one last dream, seeing his family - seeing his father; we see flashbacks of him being neglected and having to plan his mother's funeral by himself before it ends with Ren accepting it and happily screaming how Ghidorah can help him defeat Godzilla once and for all. Mechagodzilla, now controlled by both Ren and "Kevin", glowing a bright golden yellow, and its roars now changed from Godzilla to Ghidorah's, proceeds to attack civilians and the buildings of Hong Kong as well before producing an alpha call, calling any Titans in the area to his aid. Issuing his own alpha call, which Godzilla immediately recognizes as the call he had heard all those times before, and Godzilla roars back before charging Mechagodzilla. The two Titans battle and it goes about the same, but as the two fight, the other Titans begin to show up to the feud of the three alphas. As they witness Mechagodzilla dragging Godzilla across the ground before throwing him into a building, knocking Godzilla out temporarily. The other Titans bow to Mechagodzilla as their new alpha, except for Rodan, who still stands and looks over at Godzilla and then to Mechagodzilla. Rodan roars at Mecha-G and attacks him, but Mechagodzilla beats down Rodan as the other Titans avoid the robot's warpath, but Rodan wars the other Titans that Mecha-G isn't an alpha, but a killing machine like Ghidorah and Zo-zla-Halawa. The other Titans realize that Rodan is telling the truth and issue a challenge to Mechagodzilla, but Mechagodzilla simply laughs as the golden energy of Ghidorah surges across his body.

After a long battle, Mechagodzilla beats down all the Titans brutally, and Godzilla regains his senses, small drones discharge from Mecha-G's back to attack Godzilla with sonic/laser weapons. Brooks uses the HEAV's G-Crushers/Shock Anchors and the larger maser cannons to jump-start Kong's heart, and we have a little suspense as he doesn't wake up right away. When he does, we get the same scene as before, and Jia and Houston convince him to team up with Godzilla and the other Titans. The human army is ineffective against Mechagodzilla and is about to kill Godzilla, but Kong comes in to try to even the odds, now using electricity powers because of the HEAVs' weapons; at the same time, Rodan wakes up and, barely able to stand up, flies off to fight Mechagodzilla. Godzilla uses his atomic breath to charge Kong's axe, and Kong swings it at Mecha-G, but it fails, enraging Mecha-G/Ghidorah. The other Titans also wake up and manage to pile onto Mechagodzilla, but Ren/Ghidorah reveals a final weapon, unleashing a massive shockwave of electricity mixed with an unnatural wave of radiation, similar to the Burning Godzilla Nuclear Pulse, causing massive damage and injury to the other monsters. Unknowingly to Mechagodzilla, the energy discharge was absorbed into Godzilla, giving him a sudden burst of energy; before Mecha-G can give Rodan the kiss of death, Godzilla tackles Mecha-G and throws him to the ground while Kong follows behind. Meanwhile, the human characters try to destroy the antennas at the top of the mountain, but Mechagodzilla's AI is not allowing them to be turned off. When they destroy the antenna, Ghidorah loses control briefly, and right before he is about to kill Godzilla, Kong swings his axe into Mecha-G's back, forcing him to relinquish his grasp and turning his attention on Kong, who is now a threat. Attempting to kill Kong, Kong keeps Mechagodzilla at bay before looking at Godzilla, roaring, and throwing his axe towards him; with the sudden burst of energy coursing through him, Godzilla's eyes glowing red and fins pulsing, he charges the axe with a spiral atomic breath, causing it to erupt and glow entirely red. Kong swings the axe like Thor's hammer in Avengers, each swing melting his armor and body, severing limb from limb before bringing the axe down straight into Mechagodzilla's chest; however, Mecha-G/Ghidorah isn't finished and powers up for one last proton scream, but his head is grabbed by Godzilla, who fires a spiral atomic breath one last time directly into Mechagodzilla's face and body repeatedly in a blind rage.

Ren is incinerated and Ghidorah's spirit is vanquished forever as Godzilla and Kong roar in a cacophonous victory, lightning striking the battle axe. The rest of Mechagodzilla powers down and goes limp when Godzilla stops roaring and looks at Kong's axe, remembering the weapon that killed many of his species, and Kong, realizing that the weapon was made to kill Godzilla's species drops the axe and pounds his chest while roaring to which Godzilla roars back, making a "truce" between their species, finally ending the war. The ending is pretty much the same as Team Godzilla goes to help Team Kong while they watch the other Titans bow before Kong and Godzilla roaring to the heavens. Godzilla swims back to the ocean while Kong goes into the hole and the other Titans go their separate ways. We get a transition to 6 months later, and Kong, along with several other Titans, are in the Hollow Earth; the same thing as before, but I would make sure to show the axe is still in his possession, albeit outside his temple, as a show of good faith. The Hollow Earth is filling up with Titans, allowing humanity to rebuild their cities while Monarch is building more outposts in the Hollow Earth, sealing various entrances, including the one in Hong Kong.

There are 5 post-credits scene ideas I have: One is where Madison and the Chen twins witness Mothra's new egg hatching with Sacred Springs playing over the speaker to induce birth. And a second one where Chen contacts Mark to warn him about an ancient demonic Titan that was prophesied to rise from the depths of hell to destroy all life on Earth, especially Godzilla. Meanwhile, in Mexico, the Oxygen Destroyer had unearthed a pocket beneath the seabed, allowing for a race of ancient crustaceans to mutate from the hostile environments. A third one is where in a new secretive organization called "Shiragami Pharmaceuticals", scientists begin experimenting on a strange-looking bioluminescent rose behind a glass wall called "Experiment HE-1989", after which we hear the sound of Biollante. The fourth idea is that after Apex was exposed to the world, other personnel who are loyal to Ren's cause are in a secret facility somewhere in the world and are using MUTO Prime's skeleton to create a new cybernetic creation known as Gigan. The last idea is that Monarch has been getting signals from an unknown source from the depths of space and that strange meteors have been crashing to Earth for several weeks, all coated in odd crystalline structures. Suddenly, Chen wakes up next to Mark and warns him of the entity known as Zo-zla-Halawa, and how he's coming back to reclaim his throne.

Cut to black, dramatic 1962's King Kong vs Godzilla theme.

The End.