How about a feature that allows to add (sticky) notes to a message?
They are not part of the message-text, but they are linked to a message and are searchable.
When replying to/forwarding such a message the note is shown.
Use case:
- use it like a log-file for a long (ongoing) back and forth eMail-dialog
- add keywords that are suitable for me but do not appear in the message (helps me to find the message with my keywords…)
- note some vocabulary etc. for communication in a foreign language.
- note some topics to mention/avoid in the reply/forward