Simply... Once a tread that has NOT been replied too gets to be a few years old (maybe 1-2) It could be moved into the "Old Fourm Data" section. This way people can still check to see if it was suggested before, and if they wish, revive it in a new tread.
It could help keep the fourms cleaner, and really cut down on all the pages. This would especially help with fourms like the trade board... Where there are thousands of posts.
Also, the mods could have an option to move a tread into the old posts... if they lock it.