Your rearview mirror can do that. Yes. A larger concave mirror can direct more light back, but then you have to have a 3 foot wide mirror taking up space in the cabin or exposed to the weather outside the car, it would be prone to failure.
Another car idea that I'm sure would just get me into trouble is having a mic and loudspeaker so I can speak to other drivers instead of just honking. A horn lacks the nuance of the spoken word.
I know a guy who installed a large flood light on the back of his truck. He officially has it as a work light, but he uses it to flash tailgaters and people who follow him too close with their high beams on.