I don't think you understand. Let me elaborate:
When a new thread is started, it acts as a "main" thread. When a user replies, they can tag their reply using a bit of custom BBcode:
[tag="grey"/]
If no tag is added, the reply will be threaded without one.
In a thread page (probably next to the Post Reply button) will be a textbox captioned "tag". Entering a tag will display every post in the thread with that tag applied, showing a seamless conversation on a subtopic within the main thread.
To go one step further, another bit of BBcode syntax could hide the post from the normal thread view, removing the possibility of getting a misdirected reply but still showing the post when the tag is requested.
This wraps up the issue with stray topics and keeps topics afloat for the relevant people.
