I mean hey, by all means if you think a community is too hive mind-y or echo chamber-y then by all means don't join. That's the beauty of small highly customized communities - it can be moderated in a way all the members agree with, and anyone who doesn't like it can find or found a different one.
I don't know what exactly you're imagining such a community would disallow, but I feel like whatever it is, I'd agree with it being disallowed. Disagreeing with someone is typically fine in most communities I've seen, it's just hate speech or any -ism or -phobes that aren't. And that's fine.
I'm not sure I understand how this isn't already possible. Create an account on some federated platform, such as your own self hosted one, and people from any federated platform can now follow you. Isn't that already POSSE?