Currently Evolution uses "show" versus "don't show" in message header preferences. However, this is very much restricted. For instance, in most cases I would only like to see only two headers: From, Subject. While other entries, like Reply-To and Newsgroup, etc. are needed for special messages (e.g. the ones I want to reply to).
The current situation poses a dilemma: If I show all of them by default, they will simply clutter up my screen providing information I don't want at the same time; on the other hand, if they are not shown, to view the extra headers for special messages, I need to set the preference in a time-consuming manner.

Thus I think adding a GtkExpander would be a very nice solution for this. By default it just shows concentrated information of the core information, and after expanding the user-preferred headers will all be shown.

This could be implemented with ease, I just hope it's a good idea for many end users, since I just switched from Thunderbird to Evolution and I noticed some shortcomings in usability like this.