It seems to me that the abolitionist movement got an enormous jolt forward due to the American Revolution, but the seeds of it were already growing before it happened. Oglethorpe banned slavery in the initial charter of Georgia, the Somersett decision ruled the thing illegal in England, and there was already enough of a shift for the slave state-dominated Congress to ban slavery in the northwest territories.
So it seems quite possible that Britain might already have a distaste for slavery. So what would happen as Westward expansion is organised? It seems to me quite possible slavery could be banned West of the proclamation line. Is it possible that most of the Deep South could never have slavery? What would be the impact of that?