The next war will be over the same debate as there was going on in 1787 with the writting of the Constitution, Jefferson vs Hamilton. The Constitution was the compromise between those two forms of Government. When you get away from that compromise you devolve into the european system of Goverment of the elite and not of the people. Slavery in 1860 was just the fuse to the power keg, Not the power keg itself. The next War will be over some other "fuse" issue like the 2nd Amendment, or Welfare reform, or removal of unemployment benefits, or some other constitutional right, or precieved governmental entitlement "right".
To understand that Slavery was NOT the reason for the American Civil War, you have to study and reasonably answer the question of why Free men who did not own slaves would fight? You have to discover the base motivation of these common men.