Just thinking about the distinction between Riot.im chat, and Discourse forum, again. IMO Chat Rooms are convenient venues for “in the moment” discussions between two or more participants, the content of which does not need to be discoverable by others (if that sort of information arises in the discussion, it can go elsewhere). As such I think we should have a reasonably small number of chat Rooms that are used for that purpose. To be contrasted with random user-created Topics in Discourse, which will typically not be directed at specific recipients (e.g. any Cylc expert will do) and are retained in a way that is easily discoverable by others for reference in the future.
(On the other hand, maybe Riot is meant to be used as a free-for-all of rampant Room creation … can’t see how that would be manageable though; and maybe Discourse can have private topics for chat-like functionality … but I think that would be inconvenient to use, compared to just diving into an existing chat Room).