PieFed already has such Categories of Communities, it's a really nice feature. PieFed has a lot of such things actually - like hashtags, the ability to block all users from an instance without requiring admin approval, YouTube embeds, etc.

Unfortunately PieFed is not quite ready for the masses as its more foundational features aren't finished yet, like much of the times a Notification won't point to whatever caused it for whatever reason, and it lacks user tagging, and search options.

But it's nice to see these kinds of features functional already!:-)