The most plausible scenario, in my mind, is one in which the North would have set up some sort of system for promoting economic development among the freed slaves. This would have at least made the African Americans less likely to be poor and less likely to be as vulnerable to white domination.
What the North could have done would have been to confiscate at least some of the land that had belonged to big plantation owners. It could have gone to poor farmers of both races. Perhaps they could also have set up some sort of bank to help these poor farmers buy the things they needed instead of having to become sharecroppers or to depend on the crop-lien system. Finally, they could have set up extension services such as were done by land grant colleges to help farmers learn better techniques. All of these would have made African Americans more economically secure.
It is hard to imagine that much could have been done (without ongoing federal control) to give blacks civil rights. That just wasn't an idea that was in any way acceptable to most whites (even in the North) at that time. But I think that improving the economic status of freed slaves would have been a major benefit and would have led to more rapid social change.