I would like to see...
1) programmable from a real keyboard. I don't know if you have access to the keystrokes coming from a real keyboard... but if you do this would be great!
a) if (1) then... The ability to save the current configuration into a key file for later. So I can configure with the keyboard everything for game X then save it after it's all working... and call it later normally.
2) Multiple keystrokes one... and maybe seperate push and release... What? I would like to have one keyboard key that will do a few items. What I really want is to push a full throttle button that will send a F10, F11 and F8 times 2. This will show FPS, Unthrottle and change the skips in mame to 11. Then on release, I would want it to send a F10, F11, and f9 x2. So undo it. I can modify the code to do the same... maybe I can talk analog mame for an addition

.... but if I had the features, I would probably use it often,
but these wouldn't be ingame so if they where slow it would be fine with me.
3) Repeating keys would be nice... but I probably wouldn't use it... but if I could turn something on for my kids, they might like it.... especially airplane games that my daughter likes.
4) A SHAZAM key! but I have no idea what that is

... No, but really the ability to change banks based on a button would be nice. I could see doing this for kids setup (repeat keys) and my keys... then have a toggle switch on the arcade machines that my kids can flip when they want to play.
All I can think of for now...