Couldn’t any instance or app do this already? Like #peertube does videos in a way that isn’t necessarily fully federated with #mastodon. We get partial functionality everywhere and some places will have some extra things. If it is popular enough, then add it to the standard and let everyone who wants it add the functionality.
There’s many ways these communities could end up gathering over time given the features of the platform we have. The most likely in my opinion is that certain communities on certain instances will take off and gradually people will focus on those instead of the many duplicates on other instances. It’ll probably be quite a while before enough critical mass builds up.