As I've adventured in the world of custom UIs, I've seen a lot of uses for straight forward, empty frames. For example, http://www.thepalsforlife.com/~palupload/HalcyonUInew.jpg places an empty frame with a semi-transparent background and tooltip borders underneath the chatbox and the titan panel to give both the appearence of having neat borders.
However, most of the approaches for getting these effects or frames are rather roundabout. The screenshot I referenced above actually uses empty frames created in Discord Unit Frames.
So why not a very lightweight addon that creates frames for you to use for whatever you can dream up? I imagine it would just need to give you a simple set of options to name and create the new frame, set the parent, anchor and positioning, and set the type, color and transparency of the borders and background, etc, etc.