It would be nice if there is a possibility to respond to (older) messages like whatsapp and telegram does.
Maybe you could keep the subject of the selected message, pretend a "RE: " and also keep the old message body with a pretended “>” each line.
On top it would be very nice when there is the same behavoir when the contact responds to the message.
Related, but different: Subject of emails
Maybe makes useless: [Discussion] Make DC a replacement for MUAs? ; but keeps user experience