I know what I want from Cars 4, which I actually think would be totally feasible and fitting within the thematic interests of both the Cars franchise and Pixar's oeuvre as a whole:
Los Angeles. Loosely centered around Cruz's first real shot at the Piston Cup a few years after she begins her racing career. They head out to the final race in Los Angeles, and find not only a race, but also the forces of Hollywood glamor and gentrification. (This could make use of unused storyboards from Cars 3.) They also find, however, a city as dense with history as Route 66 and Thompsonville. (Maybe take Santa Monica, end of Route 66, as an opportunity for a historical montage?)
For all their investment in Route 66 nostalgia, however, what Los Angeles forces Lightning & Co. to recognize is that the heyday of the mother road was also a time of intense and tumultuous social/infrastructural change. Simultaneous with Radiator Springs' idyllic boom was the creation of the 405 and other famous Los Angeles freeways, which vivisected Latinx neighborhoods in East LA. It was a time for the Chican@ movement in the city--which would open roads, perhaps, into Cruz's past. Perhaps she's an Angelena, and her family knows well this history.
I think it'd be hugely interesting to make a movie about the impact of car culture and the US's national freeway system in a world that is populated entirely by cars. And they could reference street racing culture (complete with brief visual shoutout to The Fast and the Furious, naturally), as well as lowrider culture.
It'd be an opportunity to not only explore Cruz's backstory but Sally's as well. Sally never said what kind of lawyer she was, but given her interest in historical preservation maybe she was involved in cases related to Los Angeles development--and the reason she left was not only the cut-throat fast-pacedness of her hotshot law firm, but also the very nature of the cases she was being asked to take on.
I haven't decided what Lightning's arc would be yet, but now that I've had this thought I WANT THIS MOVIE. FUCK. And the premise totally feeds into Cars's interest in historical/present-day car and racing culture, and into Pixar's forays into bringing greater diversity to the silver screen (please hire Latinx actors, animators, and screenwriters!), and Pixar's overarching interest in environmental themes.
So. Where do I send my resume?