The idea of developing a community discovery protocol as a subset of ActivityPub is a really good idea. Something similar to DNS could work but the problem lies in that DNS is fairly centralized and controlled. Look at what we have now with the political shit show that DNS has become. I think a whole new protocol would have to be developed to facilitate this. A community discovery protocol might share community lists between instances in a federated way for example - maybe much in the same way that ban lists are federated.