Do you think Slimer will be invisible when not in play (i.e. hidden in the city)? I think that will be a neater effect than simply being lowered into the way of the ball.
Regarding the Ecto Goggles, I'd love to see a mode or two where the ghost leaves the magnifying glass, making it look like the ghost travels around and you are lucky when you capture a glimpse in the viewer (kind of like the movie 13 Ghosts). Some ideas:
1) The animation would have him leave out of the view of the goggles via the bottom of the glass... then the pin would have the flippers act goofy as planned (but as though it's the specific ghost and you can't see him), then have him return to the goggles flying upwards (and the flippers return to normal).
2) Have an roaming ghost you need to hit, so similar to the man overboard in WH20 or the super jackpot in Attack from Mars, have the target (ghost) cycle the lanes. The ghost would appear in the goggles, then leave the viewer - flying the right cycling clockwise, where he would become invisible and need to be hit in the flashing lane (perhaps the lane lights would all be white, except for the active lane flashing red), and when he flies back in the goggle lane from the left side, he becomes visible again before continuing to cycle right.
3) Some sort of ghost overload mode (a combination of both ideas above)... This might be more difficult to achieve, but have 10 ghosts you need to capture across the table. Ghosts would fly in and out of the goggles, so you would see different ones appear as you capture all 10. I'm thinking it would use more generic ghosts like the below...
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