i would set your last button as a shift key, so you can potentially have another 14 functions assigned to the buttons
here are some mame features i would suggest you then add:
UI CONFIG MENU (TAB KEY) PAUSE RESET (HARD) SAVE STATE (save your game progress) LOAD STATE (load your game progress) FAST FORWARD (useful for skipping the often slow game initialisation/hardware checks on startup of loading certain game roms)