Idea Summary
Implement (expandable/collapsible) nested table functionality natively in APEX

Use Case
Nested reports provide a much more pleasant user experience than popup dialogs or master/detail/detail adjacent regions

Preferred Solution (Optional)
Open source plugin exists readily, maybe provide such function in the framework with tighter integration into the page designer. 

an easy approach is to add a new region template as hidden report (much like inline-dialog and popup lists), use existing page designer features to customize such hidden reports and provide a mechanism to “mount” these reports as sub reports in master ones.