Hopefully there will be at least one company that will offer a hackable car. I'd hate to be locked into doing only the things the designers want me to do with a thing I own.
I doubt that it will be by design, cars carry too much legal liabilities for that. But even now you can hack your car's suspension, engine or exhaust system sometimes at the cost of loosing your new car guarantee.
I could hope that the hackability could at least extend to changing behaviors not related to safety, but I'd imagine there would be some justification for just about anything being about safety.