I realize that instances are not magazines and so on, and this analogy has technologically weak comparisons, but I think the principle works.
I do think that we will start to see communities getting their own hosted instances. A light novel/manga I read has an entire instance devoted to communities about the series, and I've seen some chatter in the selfhosted community about making an instance for selfhosted/datahoarders/FOSS in general, though we'll see if that actually pans out.
I really like the model of a community of communities being in containerized into one shared, dedicated instance.