Discourse has suggested topics at the bottom of the page. That's a great way to keep users on the site as it allows them to discover new topics. It also serves as an attractive 'footer.' Would be great to get this in nodebb either as a plugin or core feature. May need a good algo to determine what to suggest...
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Suggested topics like Discourse
Display more categories
I think esotalk has a really nice way of showing the other available categories. Here's an example:
It doesn't waste the breadcrumb div. You can clearly see where you are and are also able to quickly explore and navigate to other categories.