Well, my input would be to keep the number of inputs down as much as possible. I'm having trouble with 3dArcade right now because every key on the keyboard is mapped to do something in the frontend, and even trying to unmap them is a chore... much less figuring out how to fix it when a friend starts wiggling joysticks and it ends up with tunnel vision, a camera sunken through the floor, and all the arcade cabinets in the room half size and facing the wall.
I just think there's something to be said for simplicity in an interface... like handling more advanced actions with context menus instead of keyboard keys. One interface design I've seen that presents lots of options in a simple way is a marking menu. You hold down a button and a circle pops up with options on the 8 directions, and you just move a joystick or mouse in the direction of your option and release the button to choose it. It could be done with a spinner or trackball too, as a wheel that spins and you place your option at the top and release, that would fit more options into each menu if necessary. You can also differentiate between holding the menu button and clicking it. Holding the button with a game selected might bring up search options, toggles for different artwork options, "add to favorites", "main menu", "choose new genre / game set", options menu, "show game info", and "Show clones/play game". Tapping it might show the clones, and double-tapping might play the parent game.
Apart from that, another issue I'm having is that it's most efficient to look through cabs with the clones filtered out, but often my rom for the parent game doesn't work for some reason. So, maybe a way to flip through games by parent, but when you choose a game, instead of going right to Mame, have the clones appear so you can select one if you want.
Oh, and most important - a good, deep search to find which games are
actually available and can be played in Mame, and one that can be done easily like you can in Mame32. For some reason, I went through the steps to put only available games in my gamelist, but often choose one that's listed as available and Mame can't play it. Dunno why.

I know it's never happened in Mame32. If a game is listed as available in mame32, you can play it.