I don’t agree. If I like LOTR and giraffes I don’t want to create an account on both “instance groups”. I want to do like today and create a single account, then subscribe to the communities I am interested in wherever they are.
To me it sounds like you are sort of mixing up community location and community discovery. This is sort of the case right now because instances have a list of local communities but I think that it is best that they are separated. For example on Reddit I don’t generally find new communities by scanning the entire list of communities. I usually find them when someone mentions a related community in a comment of a community that I am already in. Or when I stumble across a community when searching the web. When you discover and subscribe to communities this way it doesn’t really matter where they are hosted or if they are grouped. You can organically discover things that interest you over time (although I agree that it can be a bit slow to start).