As to who and what, this is my theory: that Axis Powers Hetalia
and Scandinavia and the World
take place in the same universe. In Hetalia, all the nations are asleep, dreaming, and reliving all their experiences. As they reach the end, nations that are younger or less prominent will appear in droves, as foreshadowed by the arrival of Sealand in APH, due to the fact that they're not reliving as much. The oldest nations, such as China, will have the longest time of it. Those who died while in the dream include Rome, Germania and the other ancients. Meanwhile, in SATW, all the nations are envisioning the future (from their POV). Someday, when the last SATW'er wakes up, everyone will meet again.