Let me give an example: I run a Civil War forum, and we have a thread called "Grant's Orders to Naval Units at the Siege of Vicksburg"
In our forum, I would want to access this thread from as many as three forums: The Ulysses S. Grant Forum, The Naval Forum, and the Siege of Vicksburg Forum.
Ideally, I'd like to be able to view this thread from all three forums. This can be accomplished by moving the thread from forum to forum using permanent redirects, not the best solution, but it kind of works.
The feature that's missing is the "Live" part, right now thread redirects age and never update based on the replies in the thread. This means a recently updated thread with old redirects will not be viewable at the top of the thread list in the additional forums with redirects.
Another way to look at this: Assigning a thread to multiple forums. I'm not sure what way the developers would prefer to implement this, as an additional feature of "Moving Threads" or as an "Assign Thread to Multiple Forums"
One other "nice" feature to be included would be some type of breadcrumb indicator showing what Forums the Thread is shared in.
To be clear, I am not suggesting that threads be copied from one forum to another, as has been developed in a current addon, creating multiple threads.