We found the basic models in Slack, Teams and Zulip too rigid. In all such clones of the Slack design, the members of channels (and threads) are static. You are either a participant in the entire channel or you arent. But it doesnt work like that in the real world.

For starters, a thread is a personal idea. (I think this is what you mean by opt-in). My idea of a thread is probably not your idea of a thread. So the concept of shared thread (channel) is hard to fathom. Second, my thread is made of activities I need to complete for the task at hand. In the process, I will interact with as many people or as few as I see fit, all in the pursuit of my goal.

Not all the people I interact with (thread participants) are necessarily members of my team. This is where the rigidity is a real problem with the aforementioned products. I cannot know all the participants at the time of channel creation. And once involve a non-team-member user in the thread, they become unwitting recipients of noise, and they cant do a damn thing about it.

There's gotta be a better way.