Would be cool to have networked machines that lets you affect your opponents game.
For example get the right combo of shots and you freeze their left flipper for 5 seconds. You drain a ball, opponent starts a multi ball. Single jackpot that builds until the first person can get it but both players simultaneously build it. How long do you let it build up until you go for the jackpot.
Would be pretty cool. And would probably even work with many games already out there. Just tweak the code.