Give some way to map url for attachments better than using a carefully choosen relative path for --attachment-basedir ?
Change the way to retrieve attachments from the mail by putting it into the same search as the one searching for text parts (might the current way miss some attachments?).
Use message "id" as file name for --html-to instead of "identify", so that paths can be communicated (sent over email to someone else with the same mbox files) without worry about changes in mbox paths?
Please ask when you're interested and whether I'd like to have sponsoring for some of these.