Communities can only be created on Lemmy.one by an administrator. While we figure out the direction we want this instance to go in, in terms of moderation, we are curating the communities hosted here on this instance, to avoid duplicating the efforts of other communities on Lemmy and ensure we're only offering unique, high-quality content.
If you moderate a Subreddit with 50K+ subscribers and would like to create your community here on Lemmy.one, please message u/JonahAragon on Reddit.
If you have another idea for a community, you can reply to this thread with your proposal for consideration. Lemmy.one and the Lemmy federation as a whole is still quite small, so communities can't realistically get as granular as they are on Reddit yet, try to think broadly and we'll go from there. Include whether you'd be interested in moderating your proposed community too :)
You can of course always create a community on any other Lemmy instance if you are not able to create one here, and users here can follow communities from any other Lemmy instance as well.