I think each individual state should be able to choose whether they stay in the US or not. Every ten years, the citizens of the state voted. If you vote to leave, you can’t come back for ten years. It would take place at the same time as the census.
Yes, the one aspect of the civil war that I have never agreed with was that a stare can not choose to leave the Union. If a terratory can vote to join the Union, I see no reason why they couldn't vote to leave it. On that note, I would also support the idea that the other states should have the right to vote out a state they no longer want in the Union.