I completely agree with keilmillerjr. Keep it simple.
In general, my suggestion is to look at joysticks and buttons as the enemy. Every additional control you add to your control panel is going to make it look more goofy, more tacky, and less elegant. You should only put what you absolutely NEED on there.
Here's my recommended layout:

* You basically have a fighting game control panel, but there's no button for MK3/UMK3's run button. 4 buttons on the bottom row handles MK3/UMK3, and Neo Geo games. I think it's mandatory.
* You want to be able to press the start button without taking your left hand off the joystick. There are games that actually use the start button during gameplay. Having to criss cross your arms to press the start button is uncomfortable and sucky. It's not ergonomic. Even though this layout isn't symmetrical, it's simply better, more comfortable, and more functional.
* You don't want coin buttons on your control panel. Basically every game worth its weight in crap has a free play mode. Stinking up your control panel with buttons you should never need to press makes no sense. If you MUST have coin buttons, just get some of those push buttons that look like coin slots to put in the traditional coin area in the front of the cabinet. Don't mess up your control panel with unnecessary crap.
* Pause button on an arcade cabinet? Why? Arcade cabinets basically suck for any types of games that aren't, well, arcade games. You don't want to play Final Fantasy or any game you'd want to pause on an arcade cabinet.
* Do not try to combine fighting game layouts with 4-way sticks and trackballs. It simply does not work. If you look at the games in MAME, a lot of the trackball games used vertically oriented monitors, and basically every fighting game uses a horizontally oriented monitor. It wasn't meant to be. If you want a trackball, make a dedicated trackball cabinet with MULTIPLE trackballs so you can actually play multiplayer trackball games properly. 4-way sticks don't even really buy you that much in MAME anyway. You can always just disable diagonal inputs in MAME, which makes 8-way joysticks pretty decent in 4-way games.
* See how dumping the trackball gives you more space on the control panel for P1's left hand to rest and P2's right hand to rest on the ends of the control panel? You want to give people some breathing room.
Here's an image of another panel I saved from disaster to illustrate the point:
