Did a quick mock up of how I think the controls will be laid out for my arcade cabinet (not to scale obviously ;). I'm not yet 100% convinced about putting the 4-way joystick and the spinner above the 1 and 2 player button sets, by my thinking on why I think this will work is three-fold.

One, most 4-way arcade games put buttons on both the right and left side of the joystick, thus using buttons to your left isn't a huge leap with a 4-way stick (as opposed to most 8-way games which have the buttons only to the right of the joystick).

Two, putting the spinner on player 2's side means I need to change less game configs in MAME since there are less spinner games than 4-way games. Also, like the first point, some spinner games had buttons on the left (most notably Tempest).

Three, moving the 4-way joystick and spinner above the buttons instead of above the trackball (like most people do) allows me to use the trackball for games with a lot forward trackball motion (ala Golden Tee) without ramming my hands into the controls.

We'll see how it pans out when I actually do a real prototype and play with it. Luckly the I-PAC that I will be using for keyboard encoding uses "shifted" keys, thus I doubt I'll need any other buttons (yet still be able to use all the config and misc buttons used in emulators). I'm still working on the actual button keycodes for use among all the different emulators I plan to use (it's a lot more involved than I thought). I'll post those when I settle on something that works and makes sense.