most seem to be suggesting hardware advancements. what about achievements? heck they are used in video games, and even the workplace and school now (gamification), why not pinball. It would be cool to have other objectives besides points, some small examples of this being used today is:

LOTR: Destroy the Ring time
WH20: Insanity Falls shots

Getting Acheivements could be used to unlock extra Animations, music, or even modes on the machine.

They could be straighforward like "Beat the Wizard Mode", or obscure like "Beat Mode with only the Left Flipper" or "Make Shots Sequentially from Left to Right".

Stern could use this unlock feature by making an impossible mode to complete to get to the next part so you don't realize they never finished the code.

You could probably do it with the glass off.

The greatest idea I have for my personnal custom machine has not been listed here.
Excellent...

I thought about that as well. They'd have to make it absolutely impossible to do.