I’m finding that it would be much more convenient to use the Notes pane if I could select a note (i.e., open it) by double-clicking. This would be much easier than having to target and click the tiny maximize button.
At present, single-clicking highlights the note, but doesn’t seem to do anything else, so I’m not sure if there’s a future feature that will use this behavior. In any case, double-clicking the title bar is analogous to double-clicking the title bar of an app, which is of course a standard way of maximizing a window.
If you do implement this behavior, then I would highly recommend making it a toggle, as it is with app windows, so that double-clicking an open Note would collapse it. The tiny “restore / maximize” button still serves as an indicator of the state, I suppose, and as an overtly discoverable affordance, although I’m not sure if I would ever use it if the title bar itself served as the toggle. Since I appreciate your desire to avoid hidden features (e.g., multiple modifier keys), it would be consistent to leave the button.