I’m not sure if this is an original idea, but sharing just in case.
Many modern cars have push-button starters that do not require a key to start the ignition. Usually, the proximity of the owner’s fob is sufficient to start the car. The problem, of course, is that the code to open the car is the same, and (by design) it needs to work from a distance. This provides ample opportunity for hackers to intercept the code (and either open, or steal the car), even with various techniques that try to obscure the code.
It occurs to me that the process of opening and of starting the car do not need to be combined.
The fob could contain two transmitters, with different coding schemes. One of them would be used for remote entry, as in current designs. The other, which would be extremely low-powered, and only operate from a range of two or three feet, would be used to enable the ignition.