Freakin' awesome.
Ok so to 'impove' upon the skill shot idea, how about if your skill shot gets the 88 or desired speed then a scoop opens up somewhere on the right side of the playfield and takes the ball below and makes it disappear in time (kicks it out wherever). Point being that if you miss the skill shot you either just loop around, or hit some standup in the back right (old man peabody's barn) to indicate that your skill shot has failed.
As far as the cabinet, I don't particularly like the backglass but I don't have a better idea. The rest of the cabinet should be made to look like a Delorean as much as possible. Keep the blue lights as an option that only goes off at certain points to indicate time travel. The topper should be Mr. Fusion. The Delorean is so cool looking and there are so many components to it.
The back wall of the playfield should be the flux capacitor directly in the middle...maybe amidst some time circuits and whatever. If you want a more integrated approach to the flux capacitor, maybe put the clock tower in the back with some other hill valley images.