Surely this rests on European immigration.
Just have the North be anti-immigration and the South pro-immigration and not only will the south transform from agricultural to industrial but it will also have the majority of the population living there as well as the abolitionists.
Perhaps, but breaking the power of the South's Landed Aristocracy isn't going to be pretty, unless you somehow prevent them from rising in the first place....
Maybe have the troubles in the British Isles become greater, pushing more colonist to the Americas....while the Indians and the Appalachians are still an effective barrier to the west.
Or have the French to better in the New World, forcing the population of the Northern Colonies to flee south or be under the Papist's evil domination....
Could you see a New France Spaning from Canada down to Pennsylvania....?