I understand why some folks would want this, but I just don't. I don't want to interact with users from e.g. .ml so I don't go to communities hosted on that instance. But if they were joined in, you'd get the same people in News and Politics regardless of where you go.
I would pretty much be forced to find or run an instance that federates with exactly the right servers. Even then, I'll have someone talking about what u/shitfart said in their comment and I won't have a fucking clue what they're talking about. Then there are the people caught in between who see everything and wonder why people are talking like they don't see one another.
It would just be an absolute shit show. My instance is having some federation problems right now and I'll see someone say "I'm jumping on the bandwagon" but there is literally no one else [that I can see] who is taking about what they just said.
I totally get why this would seem to be a good idea, but I don't like it.
It would be great to implement client side. I just don't know how the comment system would work. Do you post to 4 communities all at once? Obviously moderation issues if it's a single comment linked to 4 servers, but if it's not then people who don't have a client that combines duplicate comments on duplicate communities would see a bunch of garbage spam.
I guess I'm not saying it's inconceivable that this feature could be done well, but the obstacles seem really tall. I can't even imagine how I would implement this.