The Pixar Theory

Monsters Inc/Monsters University

So what happens next? Humanity, machines, and animals grow in harmony to the point where a new super species is born. Monsters. The monsters civilization is actually Earth in the incredibly distant future.

[Someone wisely pointed out that in Monsters University, the college is said to be founded in 1313. If we're really in the future, then that means the monsters could have reset society and begun using their own calendar. That could mean Monsters Inc. takes place up to 1400 (or more) years after A Bug's Life.]

Where did they come from? It's possible that the monsters are simply the personified animals mutated after the diseased earth was irradiated for 800 years.

[Not during Wall-E. I would guess that it took hundreds of years after Wall-E for the animals to become monsters]

Whatever the reason, these monsters seem to all look like horribly mutated animals, only larger and civilized. They have cities and even colleges, as we see in Monsters University.

[An issue some have found is that this doesn't properly explain what happened to humans. I haven't settled on a theory I really like yet, but I'm leaning towards the idea that monsters and machines eventually forgot that they need humans and got rid of them again, not realizing their mistake until all humans died out. Another explanation is that humans just couldn't survive on Earth anymore.]

In Monsters Inc., they have an energy crisis because they are in a future earth without humans. Humans are the source of energy, but thanks to the machines, again, the Monsters find a way to use doors to travel to the human world. Only, it's not different dimensions.

The monsters are going back in time. They're harvesting energy to keep from becoming extinct by going back to when humans were most prominent. The peak of civilization, if you will. Though a lot of time has passed, animosity towards humans never really went away for animals/monsters.

Monsters must have relied on anti-human instincts to believe that just touching a human would corrupt their world like it did in the past. So they scare humans to gather their energy until they realize that laughter (green energy) is more efficient because it is positive in nature.

[An alternative explanation that fits even better that some of you brought up: The machines and monsters created the time travel doors but realized that messing with time could erase their existence and change history. So, they falsely trained monsters to believe that humans are toxic and from another dimension, making it suicide for a monster to interact too much with their world. Another issue is how the monsters seem to worry about kids "being less scared these days." It's likely that going in the past takes a lot of energy, so the monsters can only go back as far as the practice still returns a profit in energy. To them, they're just moving through the same dimension of time, but the monsters at the top know that eventually, they'll run out. This is why Waternose is so bent on capturing children and enslaving them.]

We even see a connection between A Bug's Life and Monsters Inc. via the trailer we see in both movies. As you can see, the trailer looks exactly the same, except the one in A Bug's Life is noticeably older and more decrepit, while the one in Monsters Inc. (where Randall is sent via a door) has humans and looks newer.

[Some have argued that the trailer in A Bug's Life should be nothing but dust. I disagree based on how barely intact other buildings were in Wall-E. They also bring up the bug zapper that is powered by electricity. The zapper could easily be solar powered, just like Wall-E. The bugs probably used it as a light source to signal other bugs to "Bug City." Also, the trailer in A Bug's Life never shows lights in the trailer like it does for Monsters Inc.]

That said, Monsters Inc. is so far the most futuristic Pixar movie. By the end, humans, animals, and machines have finally found a way to understand each other and live harmoniously.

And then there's Boo. What do you think happened to her? She saw everything take place in future earth where "kitty" was able to talk. She became obsessed with finding out what happened to her friend Sully and why animals in her time weren't quite as smart as the ones she'd seen in the future.