Outlook lets me edit subject lines (undocumented, there for 10 years, just click on subject line and type. Yes, there's no UI indication that it works. Yes, it works.) It even lets me edit sent or received email bodies.
I didn't realize Eudora, a program I used heavily throughout the 90s, did the same thing ...
MailTags -Eudora lets you edit received emails, which I found handy for making notes and changing the subject line. I really missed this in Mail. MailTags doesn't let you edit the email, but you can add a note and change the subject line, and more. The interface is simple, there when you want it and out of the way when you don't.
So, what's this MailTags?! It sounds interesting, but I don't see how it changes the original subject line. I do see Google's partial IMAP implementation doesn't support MailTags metadata.