In the beginning, we have Okoye asking George to watch a new member of his pack, Paavo. In the end, we have George being introduced to a new member of Okoye's pack, Kate. In a sense, the ending ends the same way it began.
Both George and Kate have suffered a great loss of a loved one (mother/brother) to someone heartless (poachers/Claire), and both of their loved ones were avenged with the killer's death (shot by Okoye/eaten by George).
Technically speaking, the encounter between Ralph and the random dog was a face-off between two very different kinds of genetically-modified wolf.
Lizzie’s Nigh-Invulnerability makes sense when one not only thinks about how her genes have been edited, but also when you think of real life crocodile Gustav.
Furthermore, the fact that she's much bigger and stronger than the other two also makes sense even without addressing how much she's eaten: George and Ralph only breathed in some of their respective samples, Lizzie swallowed her entire sample whole.
When George eats Claire, thus consuming the antidote, she is wearing a red dress, just like the random woman whom George can snatch out of windows and swallow whole in the original games.
Part of evolution means the species changing over time to be best suited to its environment. The gene editing pathogen turns it on its head. We didn't see Ralph use his spines as projectiles until later into the film, after he'd been encountering humans with projectile weapons. Lizzie spent most of her time in the water until the end, so her mutations made her increasingly suited for water and mostly based on the aquatic animals she would have encountered - hey, gills would be handy while still possessing lungs. George spends most of his time locked in cages, rather than roaming the environment, and is in situations where a combo of strength and intelligence matter.
The fact that the trio also demonstrate abilities that are clearly inspired by other animals makes a lot of sense when you remember Kate's description of the mutegen as explicitly taking traits from other animals; they cite the unstopping growing of sharks and the growth rate of whales as why the creatures continue to grow so quickly and extremely, for instance, and that their response to the radio signal is probably based on bats' ecolocation. The spines and gliding membrane of Ralph, and the tusks, gils, and multiple rows of teeth of Lizzie, are probably explicitly traits taken from other animals included in the pathogen (which Kate states included multiple variants).
The fact that the "chill pill" cures the mutated animals of both their hyper-aggression and their uncontrollable growth probably isn't a coincidence. Growing that quickly would use up an incredible amount of energy - The trio were probably starving the entire time, no wonder they started eating people.
What's George going to eat now that he's still big?
As gorillas are naturally vegetarians it's likely he'll still eat mostly plants and fruit, though clearly a LOT more than he used to. Let's just hope he didn't develop a taste for women in red dresses...
Basically at the end George is back to his normal tame self, but he's still a giant gorilla. They never cured the process only his rage, but they don't even know if he'll stop growing. If he keeps on getting larger and larger, what happens?
Claire mentions early on when going over her plan to lure the giant animals to Chicago that the aggression cure also stops the growth process, but it just gets glossed over later when they focus more on the cure stopping the aggression and noting that it simply can't shrink them entirely.
George still killed many people and caused tons of destruction during his induced rampage. The general populace may not forgive and forget that so soon.
Ralph and his pack:
Imagine how it was for Ralph's pack to see him wander off towards the pathogen container, come back growing at an exponential rate, then turn aggressive and single-handedly kill them off without prejudice.
It gets worse if you think about it; wolf packs are more like family units, with the alpha essentially being the dad. Ralph essentially murdered his immediate family in a rage-induced fit. If he happened to be the alpha, then he likely murdered his own children.
Much, much worse when you remember the mutagen took effect gradually. Just think of him coming home to his pack, seemingly normal but slightly grumpier, then over the next few days gets worse until he sprouts wings and spines and doubles in size and devours his own packmates. Yikes.
What are they going to do with the giant animal carcasses? It seems unlikely they'd be able to clean them up before they start to stink *really bad.* That part of the city is going to be a biohazard as well as a disaster area.
Worse yet: there are probably still traces of the mutagen present in the carcasses of Ralph and Lizzie. What urban animals are ubiquitous in Chicago, have been displaced en masse by the collapse of so many buildings, and are liable to come across the remains as they search for food and a new place to live? Rats. Thousands upon thousands of rats. What was the very first mutated animal we saw in the film, and what havoc did just one such animal manage to wreak on the hapless crew of a space station? Oh shit...
The government also seized data and research from the labs that made it, and while the building was destroyed, the machinery itself probably wasn't unique or irreplacable. It would take the government very little resources to replicate what they did, only now, they'd probably think they can contain it.
Given that she was Swallowed Whole by George, how long did it take Claire to die? Was she chomped to death instantly or did she go down alive and was slowly digested? (Then again, it's hard to feel genuinely sorry for her.)
Suffocated within a minute or so if not instantly killed, probably.
George eating people in his frenzied state makes horrifyingly sense if you consider that the mutated animals were intended to be used as weapons. Claire Wyden probably thought they'd be even more efficient in the battlefield if the increased aggressivity went hand-to-hand with a taste for human flesh. While the monster trio tears through Chicago, it can be noticed that sometimes Ralph appears more concerned on snacking on people than on attacking the military...
Making this worse, the animals themselves were accidentally exposed, and the only deliberately mutated animal was the rat in the satellite lab. They weren't planning on weaponizing mutant rats (we assume), but the rat was a test subject. What follows testing on rats usually? Human trials. Energyne were planning to create giant, unstoppable, man eating super soldiers.
The three cans that held the mutagen wasn't the only ones. There was tons more that we never saw make landing, but they're out there somewhere. We already know from the games that there's tons of different monsters in the Rampage universe, and these cans can bring them into its movie realm!
Confirmed in an alternate ending/mid-credit scene on the blu-ray release, where we see a mutated giant octopus starting to wreak havoc.
Consider the fan theory that the mutant creatures take on biological traits from the creatures they've been preying upon. Then go back to the opening sequence, and watch the bit where the mutant rat swipes its clawed forepaw at the glass of the escape pod window. You'll need to go frame-by-frame to tell, but... it has a thumb. Rats don't have thumbs; humans do. That freaking thing was taking on traits from the astronauts it was devouring.
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