In a way, the US already has this. Immigration is totally free between states, and people do immigrate to where they like the laws. The only problem is that t

In a way, the US already has this. Immigration is totally free between states, and people do immigrate to where they like the laws. The only problem is that the federal government is uniform--anything it does, the states can't decide to experiment with. Hence, I am in favor of moving as much responsibility as possible out of the federal government toward the states. But wouldn't it be cool if there were some broader international treaty among nations? Say, a group that agreed to totally free immigration and trade?