The current Extent Indicator functionality in the Layout view can only draw a rectangle extent. I suggest providing an option to use the rectangle center point as a means to intersect a sub-layer within the Source Frame and symbolize the features.
Example 1: current Extent Indicator functionality.
Example 2: Enhancement to allow symbolizing of features within the Extent Indicator
A mockup of new functionality for the Extent Indicator dialog is also provided.

This opens up a lot of interesting cartographic possibilities!