The current extended headers when forwarding or replying to a message could be improved by including the headers in the quoted text, and by prefacing them with the typical "Begin forwarded message" or "On month day time, user@domain wrote:"

You could tweak the current extended headers, eg, by removing either "from" or "reply" lines to save space.

And/or instead of having to toggle the preferences between showing headers or not, you could have a quick key or a reply with headers options.