The Pixar Theory
Sooner or later this was going to show up in this Fan Theories community. So here it is:
I haven't decided personally whether or not to fully embrace this as headcanon. There are a lot of points that seem to be connected and the theory incorporates time travel which biases me toward wanting to accept this theory as true.
The Pixar Theory is really an entire collection of fan theories spliced together. Each supporting argument can be an entire post itself in this community.
The Pixar Theory grows and evolves as each new movie is released. Each new movie is examined for details to help fit the new movie into a timeline.