China tries to get friendly with the ROK and as kind of limited success; they are careful about it but can only make so much headway. So China secures the Korean border at the Yalu, both to keep its border with Korea secure and to avoid a huge exodus of DPRK refugees.
SK looks at the situation and simply mutters, "Shit." They know reunification is the goal and the inevitable end result, but it's going to be a shit-ton of work dismantling the DMZ, quelling riots, building the North back up and rehabilitating the population. Lots of people will still be loyal to the Kim regime, so reuniting the entire peninsula and avoiding a DPRK rump state will be a task in and of itself.
The end result will take the better part of a generation to achieve, and in this universe, we will most likely be hearing about the economic potential of the North, still a largely poor area with Pyongyang full of slums and desperation. However, it will probably have some investment from Chinese businesses as well as companies like Samsung and Hyundai using the area for less expensive labor; people will have to learn to trust corporations from the US and Japan again, and the generations that distrust them will still go strong. Drinking Coke or owning a PlayStation will be seen as a sign of rebellion among NK youth; that is, when young people can afford them. There may even be a black market of stolen or knock-off products pervasive in NK, with Panmunjom becoming a sort of NK version of Tijuana for a while.