XML-based diagram editting (#324)
Based on your software architecture this might be easy or hard to implement.
The feature that I suggest is having an XML representation of the diagram in main window. So user can either use visual elements to create diagram or edit the XML texts and see the changes be done in diagram. Very similar to how we edit XAML files in WPF in Visual Studio.
It makes creating multiple copies of some elements very efficient and I'm sure professional users will love it.