The Theory begins with Brave (The Good Dinosaur isn’t relevant) as we see a witch possessing the ability to pass through magic doors and animals like Mor’du becoming intelligent through magic.
Meanwhile, technology begins to become aware as in the mid 50s to 60s, the Omni-droid attacks Metroville and catches the attention of Buy N Large.
On the other side of the world, Nemo and Friends manage to spook a greedy dentist and escape capture in a fish tank, a rat manages to become one of France’s finest chefs, and dog’s become capable of verbal interaction via Charles Muntz. Animals across the world are becoming more and more intelligent.
Years later, Andy Davis’s toys are, among many other childrens’ toys, harboring a deep emotion connection to their owner due to his emotional connection to them. A child’s emotions (seen in Inside Out) are capable of bringing toys to life. Inside Out also demonstrates the power of memory where that controls who lives and dies permanently in the Land of the Dead.
Even in Soul, the Jerry’s decide a human’s primary personality traits. Riley from Inside Out obviously got Joy as a major trait (Joy leads the other emotions).
In 2057, Buy N Large has polluted the Earth to the point where it’s uninhabitable and leaves small WALL-E robots to clean up while humans orbit space in the Axoim for a few hundred years. Eventually, a WALL-E falls in love with a robot named EVE and brings humans back to Earth where a single tree is planted.
While the humans were in space, cars ruled the Earth for many centuries due to a massive lack of human energy and begin their own timeline. Car versions of famous and inspiring people are believed to have existed as well.
The remaining Axiom ships that didn’t return to Earth landed on a mysterious planet where, after centuries and centuries, magic is discovered and practiced. Technology is re-discovered and magic fades away. Luckily, Ian and Barely Lightfoot (a product of some wierd evolution) use it to bring their dad back to life.
Remember that tree, which was planted at the end of WALL-E? That becomes the home of Flik and his ant colony as he recruits circus bugs to fight off the infamous grasshoppers.
Eventually, humans evolve into monsters and must use child energy to power their cities, homes, and places of work. Monsters Incorporated becomes the number one place for harvesting that energy via time travel. They travel to the 50s and 60s (when the Incredibles takes place) because crime was rampant and fear was at an all time high. Mike and Sully discover that laughter is more powerful than screams because, as seen in Inside Out, Joy overcomes Fear.
Boo from Monsters Inc. becomes obsessed with finding Sully again and uses magic doors to try and re-enter the monster world and forever traps herself in 10th Century Scotland as the Witch from Brave.
