well, the completed forum would be for finished series which still have some little activity in them
forums with absolutely zero activity for a long span of time should go in the archive, but only for a matter of space, since a whole forum which isn't used, and is in the forum index, only takes space away from other active forums
threads of completed series with no activity do no harm if they stay out, they're going to be at the bottom of the forum anyway, no one is going to see them, but if someone wants to bring them back it'll be fast and easy, without any archive movement
i linked before animesuki because that's an example where nested sub-forums actually work, so it was a good example on this matter of series getting their own forum when they grow enough
Edited by Zeando, 23 March 2017 - 05:45 PM.