I just don’t think this is a problem which needs to be solved. On Reddit it’s common to have different subreddits focussed on the same topic. For example, r/Games and r/Gaming. The only difference on Lemmy is that they’re now m/Games@lemmy.world and m/Games@kbin.social. Yeah, it’s slightly longer, but super easy to solve using UX tweaks in the front end.
What’s more, this proliferation of communities across instances is critical for a reasonable user experience because of the apparent widespread support for defederating from instances which aren’t ideologically aligned. If people get comfortable with using one uber gaming community on one instance, that instance could disappear from one day to the next because of a capricious instance owner.
There are dozens of D&D related subs on Reddit, and many of them overlap a great deal. There is nothing stopping people from creating as many D&D related subs as they like, and people have obviously done that. It’s just that you don’t visit the small ones because of a lack of content. I’m not seeing the practical distinction here. You’ll subscribe to the Lemmy communities with content and ignore the rest.