You don't need to, but the EU is that way. We don't even know what is going to happen in the real world, that's difficult to make projection in a hypothetical

You don't need to, but the EU is that way. We don't even know what is going to happen in the real world, that's difficult to make projection in a hypothetical world were the EU was built differently. For example, would the UK have voted out in the first place in such a world ? Would Turkey be a member of the EU ? Edit: Let's add a bit more info. The goal of the EU was to become the USA of Europe. Can each State of the USA determine its own immigration rule directly with foreign nations ?