Setting groups is a nice idea!
Now the left menu has an item that is my plugin (which actually is Shell commands in the above screenshot, although I had Quick switcher selected). If I understood your suggestion correctly, I would add two smaller items below my plugin’s name in the list: Basic settings and Advanced settings. They would be indented and otherwise graphically displayed in a way that a user understands that they do belong to the Shell commands plugin. If a user would click the Shell commands text in the menu, it would automatically open Basic settings. Did you mean something like this?
I think I could do this, but I’m also thinking about consistency. If some other plugin maker would want to do something similar for their plugin, then all plugins that add “child menu items”, should do it in a consistent way, so that the submenus will have a similar look and behaviour. That’s why an official API for setting groups would be nice, but I do understand if you have other things on your plate.