Shit, is this not how Lemmy is intended to work already? I'd assumed this was part of it being "federated." Clearly I have some things to learn.
Lemmy
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I mean, right now federation is in its infancy so more advanced federation options haven't really been implemented, and this isn't something you want done by default, because two communities focusing on different things might have the same name. For example, "trees" might be referring to cannabis (a joke that originated on Reddit), or a biology community about actual trees.
Makes sense not to do it by default, but I think the option to form a single coherent community across servers is crucial to avoiding platform-killing fragmentation. Otherwise what's even the point of being "federated?" It's just a bunch of separate servers.
The simplest way to allow users to merge communities like this as they please, as opposed to having the moderators choose for them what communities will merge or having the communities auto-merge, or adding the topic umbrella over communities (which I think is an interesting idea), is to allow ”multi-communities” (like multireddits), so https://lemmy.ml/c/startrek+startrek@lemmygrad_ml+startrek@mywebsite_com would display all the community's posts on one page.
Edit: To expand on how I see @Gwynne@lemmy.ml 's topic umbrella idea, each community would have a mandatory topic (not category, those already exist), that would by default be the same as the community's name (but could be edited), and going to https://instance.tld/t/startrek would display posts from all the startrek communities on all instances.
I understand this is an advanced federation feature, so I don't expect it made soon, I'm sure the devs have their hands full already, just an idea.