"IMO, folks should be able to devise whatever groups they want: it shouldn't be centralized."

I agree with this, but with a significant caveat. There will be groups like alt.WeLoveNazis, and the system will need to think about how to react to that. This is something that the fediverse is currently struggling with, I think.

To generalize, usenet had very poor tools for defense against bad actors (because bad actors were legitimately rare at the time). Nusenet would need to be robust against a wide variety of threat models.