Incidentally, I've been meaning to suggest this feature for some time.
Some of my fold-out menus have been growing and are now higher than the height of my 24" screen, so I need to scroll up and down to reach the actions. Some of the fold-outs are actually several times higher than the screen, which means a lot of scrolling. So here I would have use of additional fold-outs to sort into sub-menus to cut down the height.
Besides the scrolling, with many items in the fold-out you lose the visual overview of the actions - the list is simply too long. I "fix" this by using fake actions as dividers - but those dividers add up to make the menu even higher, so I try to not use them too much. Additional fold-outs would solve this visual issue too.
An additional fold-out would also be nice for various sorting purposes, e.g.
Actions > Composer > C > Camille Saint-Saëns
Actions > Genre+Style > Jazz > Bebop
(I use actions to set up specific rules for some fields, hence the combo of genre+style)
Perhaps an additional fold-out separator (e.g. #, ## or §) could be used/implemented?