Based on disruption theory, I think there's a good chance Apple will eventually make an autonomous electric car. Apple is currently trying to participate in the car market through CarPlay, a proprietary but still modular interface that's designed to work across multiple car brands. I'm not an engineer, but it may well be difficult to fully integrate the iPhone, iOS, and Apple Watch with multiple car brands across a modular interface. A modular approach probably forces Apple to limit what CarPlay can do -- it has to be "dumbed down" to ensure it works well with a range of vehicles.
The only way for Apple to fully exploit the ways in which it can use the iPhone, iOS, and Apple Watch to control a car's mechanics and controls may be to make the car itself. Without a clean modular interface that (1) works across car brands, and (2) can be used to control all car mechanics and controls, integration may be a practical necessity. See Concepts page and discussion of Clayton Christensen.
I also think an electric, autonomous car makes sense given Apple's desire to "make a dent in the universe" and its resources/capabilities in industrial design, software and hardware engineering, supply chain management, and battery design. Apple is a first class "maker." This kind of project would appeal to design/maker guys like Jony Ive and Marc Newson, and Newson has already designed a concept car. And when cars become autonomous, the car's controlling computer, and its ability to work seamlessly with other mobile devices, will be every bit as important as the car's mechanics.
The author owns stock shares of Apple.