A 'Monsters, Inc.' and 'The Incredibles' crossover would be cool. The monsters could visit the city where the Incredibles live. Mr. Incredible could show the monsters how to use their strength in a more heroic way on the surface world, and the monsters could teach the Incredibles about the power of scaring (in a non - harmful way of course).
Sure is! Toy Story is a hallmark Pixar film. It pioneered the use of computer animation in feature-length movies and set the stage for many other Pixar classics.
Not sure. I haven't come across 'Joy Story' being identified as a Pixar short. Maybe it's a relatively new or less popular one that I'm not familiar with.
Perhaps it means that there are elements in Harry Potter that are similar to what one might find in fanfiction focused on elemental concepts. Maybe the magic system can be broken down into elemental - like components.
I'm not sure about the most popular ones specifically, but you can start by looking on fanfiction websites. Some might feature a crossover between 'Inside Out' and 'Cars'. Maybe the emotions could be the drivers in the cars, and they have to deal with different terrains (which represent different emotional situations) in the world of Cars.
There could be. 'Harry Potter' has a huge fan base and they like to explore different concepts. Fire elementals could be added to the story in many ways, for example, as a new kind of magical creature or as a power that a character discovers. However, not all fanfictions are easy to find. You might need to search through different platforms dedicated to 'Harry Potter' fan works.