That has to be the first time in history someone looked a 4 button per player, 2 player panel and said "it's too cluttered" =P *shakes head*
LOL!
I wonder if you'd consider doing 3 buttons on side of the t-ball for missle command/etc... but doing something else on the other side (for more functionality and visual symetry sake)
Well, my wife is left-handed, so if I don't have a fire button at least on the other side for Centipede, she'll be cranky...

I'd say spinner but it'll probably get in teh way of player 2 stick action.
Thank you for reminding me! I had a spinner at one time, but it got in the way of the trackball, and I realized that I rarely used it, so I scrapped it. I should at least plan for a space for one in the future...
Maybe you could use 1 side for missle command and the other for administrative functions --> there by cutting down number of buttons up top? (just riffing here... ) but would one accidently hit "pause" or "escape" from the player 2 stick accidently?
The administrative buttons up top do add to the clutter. I need "pause" and "exit" up there at least, but the others could probably be stashed under the panel or in the coin box....
The trackball seems further away in your design (probably by design) and makes it seem like it would be uncomfortable just to use the existing buttons near the joystick for say missle command.
Yes, it is by design, for Golden Tee, Shuffleshot, Shuuz, American Horseshoes, and World Class Bowling. In my testing, the trackball center should be an absolute minimum of 8 inches from the glass for Golden Tee; 12 inches is better. On this design, the trackball center is about 9 and a half inches from the glass. In addition, if the trackball was any farther up, I'd have to spread apart the player 1 and 2 controls because of the huge mounting plate.
Here's the layout I have now:

That's a 2-1/4" PC trackball. It has a very small footprint, so I can put its buttons just above it.
My original panel had a 2" ball and a spinner. Since it was a DOS cabinet, I didn't need mouse buttons at all:

There are some limits as to where I can place some items, as you can see in this blueprint:

Here you can see where the walls under the bumpout are, including the original wall.
Thank you for your comments! I'm gonna go stare at the plans again...