One thing: I don't know if you saw the discussion about making configurable options for communities instead of having different types of communities. Making it configurable will allow communities to switch as they wish between being closed to new members and being open.

That's a good idea, could you link me to those discussions ?

Would it be difficult to implement this kind of community configuration?

It's mostly complicated because it's difficult to manage if a post is muted or not when creating it. so effectively, if we were to change the mode to "members only can post" anyone who posted before and isn't a member will see its posts dissapear.