Both Mbin and PieFed have "categories", so that you don't need to search for and find communities at all - you can simply join like "memes", underneath "Chillin", and it'll show all of them. You can fine-tune further, but hunting through All can be a thing of the past. So... it's happening, not in Lemmy per se (yet) but in the wider Fediverse it's already here. See it yourself in action at e.g. https://piefed.social/ (3 horizontal bars -> Topics).