Not sure of the cars character. I may have at least one be an oc char, maybe a Telsa roadster. I may pick one of the background sort of char characters. I'll do some research and watch a bit of the movie later.
Same with the bug's life and some of the other pixar movies I do have.
One idea I had for my toon world, which is more of a sorta linked multiverse, is for things like space ghost, the jetsons, ect there are time gates perhaps, though not obvious maybe like the star gates are from those series. My toonworld idea, when i was a kid way back many decades ago, was one could, if one knew the exact directions, could walk or ride to the herculoids earth, then go to visit thundar's, Mightor's, scooby, surper friends ect, but one would have to know the exact directions to go to go through the gates/apertures . If Wall-e characters are small there may be small ones so that they can go through to the different worlds like a bug's life ect.
Not sure how you'd do it but my original toon world idea, that I refined from reading some info on myths, rpg books, ect, is some of these gate/apertures may not be obvious so you may go through one without knowing it. & or some may be obvious. You can do it how you would want to.
Some of the gate/apertures may be non obvious, like for example the fraggles are actually traveling through 2 of these to get to the human dimension/universe, and whatever those giants are called they visit. They think it's all on the same world just different caves they go through, but one could say they are totally separate universes but the gates are not obvious so they don't know they travel through one. I've not seen many episode of the show in ages so I don't remember if they mention knowing this or not, but thought I'd use it for an example. Some gates may be obvious, how yours works is up to you.
Some gates may be permanently open, others may open only at certain times of day, month, year, or certain circumstances are met.
the fraggle rock opening in case you've not seen the show so you can understand maybe some of my example above.
VIDEO http://www.youtube.com/watch?v=BMu7WvM_zik