One thing you can do is set the auto-bump interval per category. So any open discussion is randomly picked and bumped. Go to the category settings and enter a number in this field:
Number of open topics to automatically bump daily
It is a good idea to surface some old discussions from time to time, unless those discussions are closed (or archived) and complete. It’s the same basic principle behind “Suggested Topics” at the bottom of every topic. Once you exhaust the list of topics you are watching or tracking, it’s a random selection of topics in the current category, but not too old.