I would add 2 more buttons to each player and make it swappable. That way, that panel can be your dedicated 4p panel. Then make a swappable top with maybe 2 joys, 6 buttons each. Maybe a trackball/spinner on it too. Or you could make multiple other CPs and have a fighter panel (2joys, 6buttons) a spinner/tb panel.... etc etc etc.
Also, when you said "Any examples of a top peice around?" I assumed you didn't understand the statement. Hyiu was trying to say keep the panel you have intact, and devise a way to simply remove the top piece of the box (the part holding the actual controls) Then make an exact replica of the top panel out of MDF, wood, whatever. Put whatever controls you'd like on this new panel and then you would have 2 tops to the CP box. Depending on what games you wanted to play, simply plug in the corresponding top. Voila!