Why don't modern cars have more external sensors?
Sure, some minvans have backup sensors so it beeps if you're about to run over little Timmy, but it seems like sensor technology hasn't gone anywhere in a long time.
Why not a series of like 8 sensors arranged on the outside of the car, with a range of like 4 feet or so? I'm not sure the best sensor for the job, perhaps a laser tape measure type device, it just needs to return a reasonable directional distance to large objects.
This would give you a rough sensor sweep of the car's immediate surroundings.
You could sort of visualize it in a force-field style display.
If a car or other object triggered the sensor's proximity radius, the indicator would change color.
On the dash it would be a simple display, much like the door-open indicator on some cars.
This would not be a replacement for mirrors or anything, you would not rely solely on this indicator, but it would be an added safety mechanism to alert you to pay attention to that area, in case you didn't know there was a car there.
Another similar idea, but one that might be more distracting, would be a roof-mounted camera system that would provide a composite overhead view of your car and the immediate surroundings, so you could tell at a glance if there were a car in your blind spot.