Re-reading your post before I hit submit.... I think I am just repeating what you are saying!

What I was saying:

I think the solution is "meta instances" or "meta communities" or "meta aggregators".
A community or instance that aggregates the smaller communities.
And some way for smaller communities to submit content to that aggregator.
Like, I'm browsing my instance's "all". I find a good meme that suits my "programming memes" interest. So, I submit that post to the aggregator.

Essentially like cross posting, but a community of all crossposts and everything is treated like it's on the original instance.
But as a primary feature. Where it's easy to "submit to aggregate subscription" or whatever.

But then we would get every instance with their own meta-community, and it's just a complication on top of communities and instances.