I find half the battle here is finding new places to sub to.
At the moment, none of Lemmy – and AFAIK, kbin/mbin/etc – or any of the clients provide a native way to search all of the Threadiverse. Some of that is kind of intrinsic to the distributed design, intended to help it scale, let instances be small if required. An instance doesn’t track all the activity on all the instances out there.
However, if you go to lemmyverse.net, they have a list of all of the communities across all of the instances on the Threadiverse. You can search and filter by various criteria.