If you're a Godzilla fan, you may have found a lot of easter eggs in Godzilla: King Of The Monsters referencing to past Godzilla films. If you have not found these references here are the easter eggs that you may have missed.
1. The Maser Cannons
The maser cannons were first introduced in War Of The Gargantuas as "Type 66 Maser Beam Tanks." These tanks would have a tractor tow the main cannon behind them and the cannons would shoot electricity like a giant taser. They would become a main staple of military artillery in the Godzilla series evolving from being towed by tractors to being full fledge mobile tanks. In Godzilla: King Of The Monsters, there is a scene where Godzilla is circling around and attacking Monarch's base. When O'Shea Jackson Jr.'s character was prepping to defend the base we see a glimpse of what he is controlling and they are called "maser turrets." We don't see them in action, but my guess is that they would be like the tasers that we see with Mothra's handlers at the beginning of the movie. It would only be used to stun and not kill the monster.
2. Monarch Outposts
On the Monarch Sciences website, there are containment facility outposts that are numbered. These outposts are outpost 32, 54, 56, and 61. They are all references to a certain date that each monster first made their movie appearance. Outpost 32 is located in Antarctica where Ghidorah is frozen in the ice. If you multiply it by two then you would get 64, which is the year that Ghidorah first debuted in Ghidrah The Three Headed Monster. Outpost 32 is also a reference to John Carpenter's The Thing which also takes place in Antarctica where the researchers are stationed at outpost 31. Outpost 54 is Monarch's main base of operations to study Godzilla. This connects to Godzilla's first movie appearance in the 1954 movie, Gojira. Outpost 56 is Rodan's containment facility and is located in Isla de Mara. Outpost 56 references the year Rodan first debuted in the movie, Rodan. Finally, outpost 61 is references the year that Mothra first made her appearance in her movie, Mothra.
3. The Mothra Twins
Through out the Godzilla series, the Mothra twins (formally known as the Shobijin/Cosmos) would play a prominent role to summon Mothra. In Godzilla: King Of The Monsters, actress Ziyi Zhang plays Dr. Chen. She reveals to Mark Russell (played by Kyle Chandler) during the second act of the movie that she has a twin sister and that she and her sister are third generation twins and third generation Monarch employees. She even pulls out photos of her mother and her aunt together on Infant Island, Mothra's original home. This is the reason why we see Ziyi Zhang's character at two different places at the same time.
4. The Oxygen Destroyer & Dr. Serizawa's Sacrifice
The oxygen destroyer was first introduced in the original 1954 movie, Gojira/Godzilla! King Of The Monsters (1956). This device was accidentally created by Dr. Daisuke Serizawa as he was studying micro-oxygen to help benefit mankind. Instead, he created a chemical that eats away everything organic and destroys the oxygen in the water. He didn't want to use the oxygen destroyer out of fear that it would be used as the same type of weapon as the atom bomb. After some convincing hearing about how many lives were taken by Godzilla and the destruction it caused Dr. Serizawa agrees to use the oxygen destroyer only once. To make sure that his invention couldn't be remade he burns his research papers and sacrifices himself when he faced Godzilla.
In Godzilla: King Of The Monsters, the oxygen destroyer was used like any other bomb that had a green hue in the blast. It was used to stop Ghidorah, but didn't work and instead severely weakened Godzilla. Everyone thought Godzilla was dead until Mothra comes and reveals to them that Godzilla is still alive. They discover not only the hollow Earth theory to be true, but discovered where Godzilla has been hiding in the past five years. Everyone knows Godzilla needs a speed boost on healing his wounds and decided using a small nuclear warhead. Dr. Ishiro Serizawa (played by Ken Watanabe) volunteers himself to deliver and detonate the warhead to Godzilla. Before Dr. Serizawa died, he walks up to Godzilla and says his good-bye to him. The plan works as Godzilla is now supercharged, yet it causes Godzilla's body to become unstable.
5. Mothra's Sacrifice
After Mothra defeated Rodan, she and Godzilla find themselves in an exhausted state. In order to help an almost defeated Godzilla get back up on his feet, Mothra sacrifices herself by being blasted by Ghidorah's gravity beams head on and her energy was absorbed into Godzilla. This scenario was played out in the movie, Godzilla vs MechaGodzilla II. In the movie, Fire Rodan lays on top of almost dead Godzilla and gives his energy to Godzilla.
6. Burning Godzilla
After Godzilla was supercharged again, Godzilla glows bright red, melting everything that touches. Godzilla became "Fire" Godzilla. He gives a bellowing roar at the three headed menace and releases an atomic pulse towards Ghidorah, which burns off the wings and two of the heads. Godzilla gives one final stomp on Ghidorah's chest before Ghidorah and Godzilla explode. Ever since "Fire" Godzilla's reveal, it was no doubt a homage to Burning Godzilla from Godzilla vs Destroyah. This 1995 movie was Godzilla's one last hurrah to end the Heisei series before his retirement. Godzilla vs Destroyah is considered one of the saddest Godzilla films since Godzilla dies from a meltdown. In similar fashion, Godzilla absorbed too much radiation and his body became unstable. After witnessing Destroyah kill his son, he releases a wave of atomic pulses towards Destroyah. Despite releasing so much nuclear energy, Godzilla was still on course for a meltdown and G-Force does everything in their power to reduce the meltdown to a minimum.