Pacific Rim Fan Theories

Who's to say the Breach closed for good? The Kaiju have to be coming from somewhere in the sequel. Drifting with a Kaiju has only tuned them in to humanity's plans and made the monsters behind the Breach aware of their circumstances. The last Jaeger was destroyed and the human race would be defenseless against a second wave. Knowing this, Raache on the Pacific Rim fan board believes Kaiju are merely waiting for the perfect time to re-open the Breach and strike again.

When Newt drifts with a Kaiju, he makes the ultimate hive-mind connection with the extraterrestrial beasts. According to Demetria Spinrad, after connecting with Newt, the Kaiju realized humans are sentient beings, just like them. 

Spinrad continues to speculate that the bumbling beasts aren't just a collective of instinctual alien creatures sent to destroy mankind. They believe the Kaiju are highly intelligent and had no idea that humans were just as conscious as them. When the Kaiju Otachi seeks out Newt, she very obviously doesn't kill him. Instead, she cherishes him, seeing him as the first human attempting to connect with the species instead of eradicate them.

Since the Kaiju are all clones of each other, why don't they all look and act identically? The monsters have the same DNA but totally different biological differences and destructive weaponry. WinnieTheWoot from Observation Deck believes the Kaiju have genes that can be turned on and off. Large blocks of their genome can be activated or deactivated depending on a variety of factors, like through diet or genetic engineering. This gives each one a set of different expressive genes and, therefore, different biological makeups.

After the Kaiju are eradicated, Pacific Rim's Jaegers will find an all new, much less destructive career in construction. Redditors curious as to what will happen with humanity's struggle to recover from the wreckage got an explanation from Redditor /u/GipsyDanger123.

With the monsters gone and the Breach closed, society needs to rebuild itself. Jaegers are huge machines capable of doing large amounts of work with little effort. If the U.N. rebuffs the Jaeger program, they'll need these huge bots to act as giant cranes to get massive amounts of construction done.

During the Mesozoic era, aliens sent Kaiju to planet Earth. As centuries passed, this Kaiju stayed dormant in the Atlantic ocean, growing stronger as time progressed. Redditor /u/godiebiel connects the monsters of Pacific Rim and Cloverfield into the same universe with an explanation of the events following the original invasion during the Mesozoic era. 

The Cloverfield Kaiju started out weak, feeding on Seabed's Nectar and entering hibernation. It slowly continued gaining strength until a deep ocean disturbance woke it up and initiated the monster's rampage through the streets of Manhattan. When the Cloverfield monster awoke, it sent a signal soaring through the collective consciousness of the Kaiju. Seeing that humanity was frail and unprepared for a beastly invasion, they made their way to Earth - prompting the wave of Kaiju and the events seen in Pacific Rim.

High-speed weaponry won't work against the Kaiju. Nukes, lasers, missiles - they're all totally useless methods of defense against these alien monsters. One Redditor delves into why the sword was the only weapon that worked with such sufficiency, speculating that the Kaiju simply weren't expecting the Jaegers to dish out melee combat. 

As the Kaiju grew smarter, they genetically engineered themselves to be resistant against anything that moves over a certain velocity. Their resistance to fast-moving weaponry also explains why the government hasn't gone ahead and set up a range of high-powered firearms around the Breach.