Piefed has a multireddit like function in personal and public feeds. It helps a lot with similar communities.

The potential for abuse or culture clash comes when these disparate (yet related) communities are combined.

From a user perspective it may make sense to expose "related discussions" that you can browse to. Keeping the discussions separate yet linked could be a workable compromise.

So if I understand you, Lemmy would benefit from a grouping above /c that’s not instance related? Like Usenet had the rec.cats group which spawned rec.cats.siamese? So maybe a /u universe? If I wanted to read about television I could go to u/television which could merge feeds of c/television@piefed.social and c/television@lemmy.world and any other c/television@.? I could see possibilities of abuse but we shouldn’t let the perfect be the enemy of the good. Maybe a /u could be an opt in thing with a manager of some kind to prevent unrelated /c’s from randomly joining.

Yeah, this would be good in my mind. I don't even care if it's just the communities my instance is federated with only; I would imagine I land with an instance that shares my values, so their federation I'd be OK with (hypothetically). I just don't want to feel like I'm missing out by being somewhere else and not knowing/being aware of a portion of the community. I want the discussion just as much as I want the information presented.