I miss the regional/local subs. While there's technically a few for my region spread across different instances, none of them have much, if any, activity.
Of the more "niche" communities, the one I miss the most is r/fosteranimals. I started one here, and have posted a few times, but given the already limited audience, plus my inexperience at growing/moderating a community, and my tendency to be mostly a lurker, I have not managed any engagement in the community beyond up/down votes.