The challenge is to have the oldest long-term continuity for one or more settled agricultural society and have it or them survive up to our 19th century. I am excusing political continuity but cultural permanence is essential. The culture (and religion etc.) can mutate over time but maintain its main characteristics that denote a sense of historical permanence. So the Egyptians under the Ptolomies and Romans (before Christianity) would be an example.
Bonus if the alt great powers are all descendents of these older societies.
Bonus if the alt great powers are all descendents of these older societies.