Probably only good for the modern Office file formats, but being able to display the CustomUI (2007) and/or CustomUI (2010+) XML files, with markup, would be useful. The XML would need to be loaded from the last-saved copy of the host document using the Packaging API.

XML can be syntax highlighted with AvalonEdit.

Nice to have would be navigation links from XML-defined callbacks, straight to the VBA callback implementation, and or showing callbacks without implementations.