There are situations when a certain member "takes care" of a specific topic (as the host of a debate, or the Dungeon Master of a roleplay, or even just regularly updating an announcement topic), but is not actually part of the board's staff.

Creating a new forum dedicated to this thread is a bit over the top, especially when there are many such threads.

This would make it useful to be able to give moderator privileges not just for an entire forum, but also for single topics.

This would be so amazingly useful for my forum. Perhaps allowing moderators and admins to assign others to moderate single topics (or multiple topics using the bulk-moderation features). Obviously, I'd like this to be a permission setting in moderator control. :)

Yes, giving this tool to forum moderators would make it even better. The forum mod could appoint "sub mods", making it easy to delegate authority.

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On that note, an extra privilege in the permission mask for something like "Topic starter can moderate topic" wouldn't be bad either. That privilege would allow everybody who starts a topic to moderate what is posted to it.