Episode 4 of the Inside Pixar documentary short series on Disney+ focuses on script supervisor Jessica Heidt and her work on trying to improve the gender balance in the studio's films on the script level. So far, this work has been focused on increasing the number of female characters in recent Pixar movies and working to achieve a more even balance in dialogue between male and female characters. One line in the episode, however, implies Pixar is planning to go further in terms of addressing gender diversity, with Heidt promising that Pixar's upcoming slate of movies will feature "characters of all genders."
Considering Heidt explicitly acknowledges non-binary people's struggles for representation earlier in the episode, the "all genders" phrasing seems like practically a confirmation that there will be at least one non-binary character in an upcoming Pixar film. Considering Pixar movies didn't even have any openly gay characters until this year, this would be a huge leap forward in terms of the studio's LGBTQ representation.
Some have interpreted Riley, the protagonist of Inside Out, as potentially non-binary due to being the only character in the movie shown with both male and female emotions inside her head, while other characters are shown with either all-male or all-female emotions. Director Pete Docter told Cinema Blend the reasoning behind the gender balance of the emotions was a combination of narrative economy and the idea that kids' identities are more fluid in general.
Beyond that, it's hard to point any even possibly non-binary characters in Pixar's filmography thus far. In cartoons, the most common form of non-binary representation until very recently has been non-human characters for whom gender simply doesn't apply (think the robot BMO from Adventure Time or the shapeshifter Double Trouble from She-Ra and the Princesses of Power). Pixar's form of anthropomorphization, however, has always involved heavy gendering, so cars, robots and even volcanoes are clearly "boys" or "girls."
Theoretically, Pixar could simply make the gender-coding on one of these anthropomorphized characters more ambiguous and call it a day for introducing non-binary character. Ideally, such representation will be a bit more explicit, and it would be really great if they introduced a non-binary human character, given how limited such representation is in animation.
Animated movies take a long time to make, so it might be a while before we even see any non-binary characters in Pixar movies, but hopefully, Heidts' promise that "all genders" will see represented in future Pixar movies becomes a reality.