A second channel type - threaded channel - would help keeping the overview in channels dedicated to discussing many topics in parallel. In these channels under each top-Level-message sits a button „reply“, which Posts automatically in the message‘s thread. For creating a new top-Level-message one has to click on „new topic“ or the like, which would replace the current „send“-button.

No complex user-roles, rather simple UI-changes, still much better than at the moment.