It would really enhance my workflow to be able to generate links to individual emails by Message-ID that Thunderbird could register a custom-scheme link handler for. So, something like:
There would be something in the message view / message list interface to right-click and "Copy MessageID URL", and a systemwide custom URL handler to respond to those links. It wouldn't matter what folder the message is in as long as Thunderbird has all the folders indexed.
Then we could paste those links into our todo lists, our calendars, documentation, or any place that we want to reference a specific message from. It's something straightforward enough that other email clients could implement the same type of thing so it's not a proprietary lock-in to Thunderbird.
And since it's keyed to the message ID in the message header and not something only in a Thunderbird database, the links would survive, say, a Thunderbird reinstall with a new profile. And for messages that were sent to multiple recipients, that same URI would find the message in their own account no matter which of the recipients used it.