I too am no expert however - possibly an alliance, or at least a trade agreement ,between the Confederacy and the UK. The British mills were keen to get southern cotton and other agricultural products and saw a market for manufactured goods. The North was becoming an economic rival and this would have helped put a spanner in their works.
Would the USA have felt threatened by such an alliance, and would a new war, dragging in Britain and other Euopean powers have started? I think there would be every chance.