OOh, it's simple (for me, I'm not sure about you huhu): the ability to add buttons linking to external mel scripts and internal mel commands. If you make the ui permanent, and customisable to that extend, that would be terrific.
I would use it if it was a permanent ui, not something you have to click on a button or hotkey to bring up. Also, to me its nice but limiting it to modifiers only is a shame, there's potential for external scripts/mel command here.... /me think silo.