When you’re viewing a chat, would it be possible to add a menu item, “Set Current Subject,” which causes every message you send to have a particular subject line? And then from now on in small text at the top of the screen, under the group name / recipient name, there would be a small bit of text saying, “Subject: [Whatever]”
That would make it so you can have a fully customizable subject, but still maintain an instant messenger-like feel.
I personally am not very interested in having a standard format that I can’t change. I would like fully customizable subject lines, if possible. This would allow me to be less self-conscious when sending quick messages to traditional email users.