If the orginal USA had become known as a Confederacy, you could have had the Southern side forming a Union in 1861-same Civil War, but with the Union side fighting partly to keep slavery instead of OTL's Confederates being the pro-slavery side.
There were some short-lived secessionist movements in the 1830's-1840's, based in the northeastern US. If one of those had actually come about, the North would have potentially been the Confederacy and the South the Union.