How about using transparent tv technology as play field glass

You would be able to add all kinds of effects that wold be right above the playfield and only show them where you want them.

You could have pop up scores like zen, add special effects to a dragon to have it breath first... maybe water effects in white water....

You could black out portions of the screen to make people play blind... or add a fog to make it harder to see the entire playfield..... could make tutorials or more interative instruction in where to shoot next (as opposed to insert)

If it was a spooky theme you could have ghosts slowly appear and float around

I have seen this tech used in some redemption games and it was pretty cool.

Some challenges to consider.....how do you handle the distance between the playfield and the glass... angles may not always line up depending on height of player.... cost, how to engineer as sliding out lcd monitor is quite different then glass... how do you protect monitor from damage....

example of tech, no affiliation with this video

Basic idea is put the screen on top of the playfield instead of under it.