>How about adding a "Reply in a new thread" button next to the Reply button at the bottom of a message? This will offer an quick option when you want to respond with a message that is off-topic, without polluting the current thread.
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>The "View previous message" and tree hierarchy buttons would still show the new message as part of the old thread structure, but you would not see the new thread from the original thread.

If such an option would be available, it would, as you say, start a new thread, but that would mean that the new thread wouldn't have any relation with the previous thread.