Some comments/suggestions I'd make are:
1) You'd save yourself alot of storage space, and money, if you went to (6) 6" panels for the joystick/button combos.
That would also let you build 1 button, 2 button and 3 button panels to use for games like
Asteroids, and
Space Duel.
2) You need 4 buttons for
Star Trek, if you want to play it with the spinner panel.
3) I would make the trackball panel have just the ball, and use a button panel next to it.
The buttons on the top like that are going to be really uncomfortable to use.
4) Three buttons should cover you on the 4-way, unless someone else knows of a game I don't play that uses more.
5) If you make the spinner separate from the buttons, you could put a joystick in place of the buttons for
Tron/etc...6) You may also want "regular" two-stick panels for
Robotron/Smash TV/Total Carnage.
7) The depth of the panels is a necessary evil if you want to play GT on it.
8 ) I've found the three button configuration like
this more comfortable than the arched one, like you are showing.
9) If you do drop to 6" panels, I would recommend pushing the joysticks to the outsides of the panels like
this.
It lets you get 9" between centers on the middle ones, which is close to the distance on a factory Robotron.
10) You might want to add a
Defender panel down the road for people who DON'T suck at it, like I do.
It's one of those "Oh My God!, you have THAT" panels.
11) The 36" x 14" panel should let you squeak a two-wheel driving panel in at some point too.
Having the extra depth that you do will really help, in that you can cock the wheels off to the side slightly.
That's about all I can think of.
Let us know how everything turns out for you.