I have looked at the Addons available in regards to Threads but they do not address the fundamental issue of being able to manipulate Threading.
By this I mean you should be able to:
1. Select an email and make it a 'Root' Thread so that subsequent emails in response (be it sent as a reply or received as a response) will treat that email as the beginning of the Thread.
2. Select an email and move it to be threaded to any email with an earlier time stamp.
This could be an email already part of a Thread but it would be a 'branch' of that Thread. See screenshot attached which is from the Virtual Access email software.
3. Unthread an email from a Thread, so that it became a stand alone email, unthreaded and not a Root email, meaning subsequent responses will not be threaded to it unless the user chose to physically thread an email as in 1 & 2 above.
4, Any other Thread manipulation you can think of, provided 'Threading' is selected as being operational.
5. After 'Moving' or 'Copying' an email from one Folder to another, be able to manipulate the threading in any way mentioned above.
Hopefully this could be a functionality in the upcoming release of 'Supernova'
I feel so passionately about this that I would be willing to donate a couple of thousand or more $s if it fully came about as part of the app and not some addon which might fail with new releases.
Most of the above is a copy of a reply I made in the 'Discussion' forum regarding Threads.