Currently, in List view, if you click on "Group by:" or on "Subtasks:" the same action happens as when you click on the bit following the ":". For example, with "Subtasks: Hide", clicking on "Subtasks:" or on "Hide" both have the same effect of opening the menu.
My suggestion is to change it so that clicking on the current setting (ie, what is displayed after the : which is "Hide" in the above example) acts the same as it does now. But that clicking on "Subtasks:" would cycle you to the next option in the list (and clicking on "Group by" would do the same for its list). This would be a much faster way to cycle through these options. We would still have the ability to open the dropdown menu (by clicking on "Hide", for instance, or whatever the currently displayed setting is), but we could quickly toggle through hiding and expanding subtasks (for example) via my proposed method (as opposed to the current option which is always to have to open the dropdown and select something).
Thanks for considering my idea! :)