Keeping a median or mode of post times with the topic will be an indicator on how recent this topic is. Doing it on the last [n] posts will be a measure of how fresh the topic.
In addition to category up/down bumps, sort by freshness.
A much-linked topic may not be fresh at all. Just very relevant during a particular period of time in history. Other than faq’s ans howto’s, which can simply be bumped.