I think that's affirmatively a bad idea, even given a solution to the UX problem. Maintaining a healthy discussion forum requires the ability to terminate bad discussions that are causing problems, and making decisions effectively requires that there be a seam-ful distinction between the thread where a decision is discussed and meta-threads where someone else in some other context wants to talk about the same issues. I see where the intuition for your idea comes from (I can't just declare that my friends have to stop talking about a road trip because I'd prefer to ride the train, I can pull someone out of the circle for a side conversation freely), but it only works in closed groups where all participants are invested in their reputation and there's no clear decisions to be made.