Currently, when you double-click on a subcomposition node, it opens the subcomposition in a separate window.
To reduce clutter of multiple windows open, it would be better if double-clicking opened the subcomposition in the same window (with some animated transition), and there were a toolbar icon to back up to the parent composition. There could still be an option to open the subcomposition in a separate window if desired.
When the composition is running, you should be able to dive into the subcompositions and use port popovers and Show Events mode, just as you can in the top-level composition.
Unlike Be able to display ("expand") a subcomposition within its parent composition, for this feature request, the subcomposition would take up the entire window. You would not be able to scroll around on the canvas to see the parent composition. Instead, you would use the toolbar icon to navigate up a level.