As existing mailing programs can receive multiple different subjects, I think sooner or later group chats will need to support receiving emails with changed subjects without prompting (emails from known contacts).
What if emails would be assigned to chats by their From: and To: fields, but not the subject?
It could mean incoming emails result in group name changes, or not. For mailing list support it seems the group name could preferably stay the same, using what is often mentioned within brackets [Listname] Subject...
And later maybe a “subject” filtering tab in the options available by tapping the chat’s title bar.