Just dropping a couple more ideas I had in here for safe-keeping.
Unlike the kind of “progress bar for your forum participation” already suggested, I figured this system should also encompass “anything interesting you might have missed”. This kind of info is best done in bulk instead of a steady stream though, or else it’s just another New/Unread that you’ll be reading intently every time you visit.
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Trending topics you might have missed (i.e. topics high in Top that you have not read)
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Topics matching saved searches that you haven’t read yet.