For the pause menu I would love to be able to save/load game states, switch discs, resume, view manual, and have a universal quit game option that worked out of the box with all emulators.
Custom startup splash screen/video for launchbox would be awesome.
Ability in launchbox to have an ambient audio file playing and separate music tracks with separate volume controls for each
Bezels would be cool if they were added to the gamesdb and not something you'd have to get from another service outside of launchbox.
Sorry I don't really have anything specific for BigBox, I don't have an arcade cabinet so I don't use it.