For this part I was more thinking about encoders to keep the four motors running at the same speed - though even that won't keep the thing in a straight line of course, as wheels are never the exact same circumference and will never encounter the same amount of slip. At least it allows for a measure of control of the speed of the motors.
The position sensors were more for the third paragraph.
Probably that's also the moment that the Arduino gets pushed aside and replaced by a RPi.