Could, for example be added as an optional split view for the Stack Trace view.

1. Enable the callees view (by selecting it on the context menu - "show callees" method, or by clicking the corresponding button on the toolbar)..
2. A pane appears with a second tree to the right of the stack trace.
3. Whenever a selection is made in the stack trace, the callees of that method is shown in a tree to the right.