I think this is being worked on / discussed in the background for how to best implement this. There are a lot of complications when you get into the details of how it should work

In the meantime, another perspective is that even on centralized platforms there will be multiple communities for a particular theme. Especially at first, there may be a few communities for something before one of them will win out as THE community for it.

That's already happened here for a few communities, where there is ONE main community for a topic.

For others, it's still in the early stages or (like on Reddit) there are multiple concurrent communities for ideological reasons.

The exception would be if something is created or thought of on the platform, in which case there may only be one community for it from the start. For example, !taneggs@lemmy.ca was a fun idea that started here and has grown into a solid community now (thanks to @DaMonsterKnees@lemmy.world's regular posts 😊).