A couple of suggestions (for the Menu Plus add-on?) if you’d stuck for ideas- which I’m guessing your not!:
1. How about giving users the option under the customizer to add a variable left indent to all sub-menu items when in mobile menu view to make the sub-menu items more easily distinguishable from the main-menu items, instead of a long vertical list (see image below).
2. You could also give users the option to add a current menu indicator (in addition to the existing colour and background image options), such as an arrow on the left or right of the current menu item.