Slack-style chat where people can reply to each other in branches. They can go on endless tangents (sub-thread after sub-thread). A mind map of topics is created on the fly.
Other participants can mark a reply as "gone on a tangent" (sub-thread created) so that the original poster can see that they went off-course from the main topic. Nothing wrong with that, but in case the poster missed it others can point out that the topic has now been broadened and branches created.
People can pursue every sub-thread or keep focusing on the main topic. They could pursue some sub-threads while leaving the non-interesting ones to die out.
Toggle the view between mindmap and chronological. Possibly a hybrid of the two where the general overview is a mind-map while the zoomed-in view is chronological.
See a small map overview of the chat in the corner of your screen. See it branch out live as it happens.