Some of these theories make absolute sense, and others... not so much. We'll let you decide for yourself which ones sound accurate!
If you'd like to read about the theories more in-depth, then check out the sources for some great explanations and detailed analyses.
Please be aware of possible spoilers below.
The tree sapling that Wall-E has to take care of becomes the tree in A Bug's Life. That explains why there aren't any humans in that movie, because the Earth is still repopulating.
Carl from "Up" died in his sleep, right after being evicted at the beginning of the movie, and everything that happens afterward is his journey to heaven. Russell is his guardian angel.
Still with me?
Russell needs to help Carl, in order to get his last badge - or - earn his wings. "I could help you cross the street. I could help you cross your yard. I could help you cross your porch... Well, I gotta help you cross something." This references Carl crossing over into the afterlife.
The theory also suggests that Russell is Carl's guardian angel, because he and Ellie were never able to have a kid.
There are obviously some plot holes, like the ending of the movie (I won't spoil it!) but it's interesting to think about.
When Boo grew up, she set up to find Sulley again. She used the doors to travel the world and through time, and ends up in Brave where she discovers the origin of the Will O the Wisps. She believes these wisps will bring her back to Sulley.
If you look closely, there's a carving of Sulley that would hint that she's Boo. There's also a carving of the Pizza Planet truck on her desk. How would she know about that unless she had time-traveled to see it?
This also explains why every time the door closes, she disappears - they're the time traveling doors from Monsters Inc.
With the way Andy's mom looks, and the hat that Andy wears, it's safe to assume that Andy's mom is Emily.
Andy's hat is the same hat from the flashbacks to Emily's room, except that it has a faded ring where the white stripe used to be - almost like it had been in storage for a long time.
Emily's room is also dated significantly older than Andy's, hinting that Emily is now old enough to be Andy's mom.
Pixar sure does love to tie things together!
Buy-n-Large, the corporation that controls everything in Wall-E, takes ownership of Carl's house in Up.
Think about it: a corporation is expanding the city, and thus Carl is forced to give up his property. In Wall-E this corporation is responsible for wiping out life and polluting the entire Earth, because of technology overreach.
Also important to note, is that Toy Story 3 takes place around the same time as Up. We know this, because of a postcard on Andy's wall that was sent from Carl and Ellie. In a screenshot in Toy Story 3, you can see BNL batteries in Buzz - affirming that this company exists in this timeline.
Monsters, Inc. takes place hundreds of years after Wall-E. The monsters are mutilated animals, after being exposed to hundreds of years of radiation on Earth.
This makes sense except that it doesn't explain why the humans who came back at the end of Wall-E aren't around anymore.
When Randall is practicing scare tactics, one of the backgrounds is the same as Andy's room's wallpaper.
Some theorists also believe that Edna from The Incredibles gets the idea for Violet's invisible super suit, because she encountered Randall.
Nemo is actually Latin for "no one". Therefore, the film becomes "Finding No One". Which is true, if you believe the theory that Finding Nemo is about a grieving clownfish who goes on a psychological journey to accept the loss of his entire family.
He meets many friends along the way, who teach him how to live in the moment (Dory), be chill and easy-going with whatever life throws at you (Crush), and that you shouldn't worry about danger being everywhere (Bruce).
It ends with Marlin sending his son off to school, as a metaphor for accepting his son's and family's deaths.
The five stages of grief are also easily mapped out in the movie:
Denial: not wanting Nemo to go off to school.
Anger: yelling at Nemo.
Bargaining: Marlin does this throughout his journey across the ocean.
Despair: seeing Nemo get flushed.
Acceptance: when he finally learns to let go.
Edna knows that capes have been the downfall of countless super heroes in the past. So when Syndrome comes to her for a new outfit, she disregards her own "no capes" policy and gives him one, hoping that it will be his weakness and lead to his demise. Did she plan this all along?
Chef Skinner from Ratatouille tells Charles Muntz from Up that animals are intelligent. Skinner knows that rats have thoughts and goals, and he relays this information to his villain friend Charles. This leads to Charles' invention of the dog collar in Up, which translates the thoughts of Doug.
The trailer from Monsters Inc. is the same one from A Bug's Life. If you believe that there aren't any people in A Bug's Life because it's a post-apocalyptic world after Wall-E happened, and if you believe that the doors in Monsters Inc. cause time-travel back to when humans existed ... then this theory makes sense.
This theory suggests that Bing Bong isn't dead - he's actually a monster (from Monsters Inc.) that Riley interacted with when she was little. She probably thought that he was just imaginary.
Bing Bong is also awesome at making Riley laugh, which is how the monsters get their energy in Monsters Inc.
So does this mean he's still alive and well?
Boo from Monsters Inc. (now also known as the witch from Brave) has been planting easter eggs in all of the Pixar movies, as a result of her time traveling through the doors to find Sulley.
This tree shows up in a lot of Pixar movies, and is the result of strange magical energy or inspiration for characters. Listen to this:
The doors in Monsters Inc. were made of wood. It was this wood that had the energy to travel back in time.
This tree is the tree that Wall-E saves, and according to the theory it's also the same tree in A Bug's Life. However, that tree also bears a wild resemblance to the tree that Carl and Ellie love to visit in Up.
In each of these movies, the tree is significant: it's the source of Flick's ingenuity in A Bug's Life, and it's the source of Carl's ideas of using balloons to make his house float.
The family assumes Jack Jack doesn't have any super powers, until he turns into a fireball at the end. This must mean that Violet and Dash had super powers as soon as they were born!
This one's a little out there... but it kinda checks out.
Carl Fredricksen from Up was married to someone else before Ellie. And he had two kids.
Carl was married to a woman named Emma Jean, and they had two daughters: Jennifer and Jill. Jennifer is Andy's mom in Toy Story, and Jill is Riley's mom in Inside Out. Carl divorced Emma after a couple years and married Ellie, but he and Emma still remained good friends. Emma sent him a post card, which can be seen both in Up and in Toy Story on Andy's wall.
There is also a photo of Ellie on Carl's wall in Up, and that same photo appears in Inside Out - hinting at some family connection.
So there are a lot of plot holes obviously, but it's still quite interesting and kinda makes sense. The Theorizer goes into much more detail about it here.
This theory is pretty much confirmed here.
Andy's dad died from polio. This is why we don't see him in any of the movies. The reason why we don't see any pictures of Andy's dad? We actually do.
The house that Andy grows up in is actually his grandparent's house. When Andy's dad gets sick with polio, their family moves into his father's parent's home for his last couple of months. The photos on the walls are actually of Andy's father as a kid.
This also explains why Andy moves: they're moving back to their own home. The moving truck also doesn't have much in it, because they only lived in that house temporarily.
Boo from Monsters Inc. And Riley from Inside Out go to the same school.
In Finding Dory, a class is seen going to the aquarium. In this scene it's easy to make out Riley as well as lots of kids from her class at school. Cool, right? Just wait.
In Toy Story 3, the caterpillar room features lots of little kids playing with toys. This gets super weird when you realize that these little kids look exactly like the kids in Riley's class at school, only 8 years younger... and the timelines line up perfectly.
One girl in the caterpillar room looks just like Boo from Monsters Inc. only older.
So are they in the same class at school? Seems accurate!
Boo is a super. She appears and disappears throughout Monsters Inc. so this theory is plausible. When she goes missing for about a day and then returns, it seems as if her parents weren't even worried that she was gone - her bedroom should've looked like a crime scene, except that everything looked the same when she got back. But if Boo's parents are also superheroes, then it makes sense why they wouldn't have gone to the police: Boo disappearing is a common occurrence.
So, then, who are Boo's parents?
Boo's mom is Violet from The Incredibles.
I guess we'll find out when The Incredibles 2 comes out!
Darla from Finding Nemo is the daughter of a rock star.
Her shirt says "Rock N Roll Girl". She also loves to sing, and you can hear her singing in the background of the dentist's office scenes.
In Toy Story 3, Darla appears on the cover of a magazine. How is she famous? Well, she's not. It makes sense that she's the daughter of a famous rock star, and she was featured in an article.
It's sketchy... but it could definitely be true.
The United States Government gives spies super powers in The Incredibles. This is how Mr. and Mrs. Incredible came to get their powers, but their children received their powers through genetics.
In a lot of places in the film, the Incredibles display incredible abilities which would suggest that they went through extensive espionage training. Situations such as Mrs. Incredible flying a plane, infiltrating Syndrome's base and recognizing the monorail system as a weak spot, and recognize the signs of short-range land based missiles.
This youtuber has a detailed analysis here.
Carl and Ellie from Up write to Andy in Toy Story 3 to warn him of the animosity between inanimate objects and humans ahead. Objects, such as the toys in Toy Story or cars in Cars develop intelligence and personality, which is a danger to humans.
Carl and Ellie plan on living in solitude to protect themselves from this impending war.
Dory was bred in captivity, which is why she has memory loss. This is a sound theory, because there are always problems during selective breeding since researchers can't perfectly recreate the ocean in a tank.
It's also considered mostly impossible to breed surgeonfish (Dory's species) in captivity. The only place this has ever happened was in the Oceanic Institute in Hawaii (this setting is very similar to the Marine Life Institute in Finding Dory!). The researchers bred 40,000 eggs and only one survived - this is probably what happened to Dory. So the fact that she survived is amazing, and it's a little understandable that she was born with some complications.
Riley in Inside Out has different gendered emotions, because she's learning these emotions from her parents.
In Riley's dad's head, anger and fear appear to be in control of his emotions. This is probably why Riley would associate those emotions with her father. In her mom's head, sadness and disgust are more in control.
Joy is Riley's main emotion, so it would make sense that this would match her gender.
Anyway, who knows - it's just a theory!