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You must create a scenario where the eastern and the western empire survive to modern day, holding only greece and Italy respectively.
Secondly, the timeline must be as close to OTL as possible, meaning you need a nation of france, britian, holy roman empire and so on. The crusades must happen in some way or another, there must be a 100 year war. Meaning there should be as little butterfly effects as possible.
Technology development also needs to be similar to real world time line.
Which means you have take our time line, and modify it by adding in two nation state such as the western and eastern roman empire.
Sorry about this, I have posted it in the wrong section of the board.