It'd be great if, when adding a dropdown menu, you could stipulate whether the item is a choosable item, or a optgroup heading.
IMO this could be done in one of 2 ways:
1) adding a clickable icon (alongside the existing + and - icons) to toggle optgroup or not
2) adding a parameter within the input itself... like by wrapping the text in [square brackets], it becomes a optgroup
Just a thought :)