This is kind of a crazy idea... I'm trying to figure out how to accomplish this functionality for a plug-in.
I would like a widget that can encompass other widgets
This widget would allow you to click and drag other widgets inside it on the side bar.(similar to how the widget side bars work only inserting it into another widget instead of sidebar)
If there isn't an easy way to accomplish this then what I would probably do is have my plugin crate a new widget area and then the widget would display this widget area inside the widget when inserted.
Kinda confusing, I know... but when I get the plugin written it will make sense.