What if during Russia's siberian expansion they conquer the Manchu and the land north of the wall before the Manchu conquer China? I guess the Ming would be dealing with their own chaos and decline so the Manchus are on their own but it also means the Ming would get replaced by a non Manchu dynasty, not necessarily Han. What I don't know if the russians can actually do the thing: they are really far awayvfrom europe and the Manchu are not simple nomads on oppen steppes, they are sedentarian. Of course if Russia is successful they could get a LOT of good agricultural land in the far east and easy access to the port of Vladivostok, which means early russians in the west pacific.