If there is a universal symbol for Pinball, it would be a Pop Bumper.

Junkyard is the only game I can recall off the top of my head that does not have a pop bumper AND the game is still fantastic.

I think No Fear is another. But seeing they are iconic they still need to do what they were designed for, and that was to ensure some sort of randomness, so you cannot anticipate what is going to happen next. As retropin said, what's random about bouncing around for a minute and then being forced out in a controlled way where you then will know what happens next.