As far as the original topic though, I would be against going into much detail about human migrations, as it is best to leave that the the individual nations.
How so? Joining this Organisation means joining, and agreeing to, a
common geophysical simulation. A nation joining this map joins a world-in-progress, a whole world, and everything that is in it from coastlines to mountains to rivers to forests to deserts ... and to backstory. The only way to have a truly common simulation is for every participant to accept this and build their own story on the foundation. If you can't do that, then you don't belong on Giess.
Nobody sits down to a game of Monopoly and says "OK, I don't accept that Mayfair (Boardwalk) is the most expensive property, I'd like to play my own arrangement of streets." The GSO should be the same - join, and play by these rules and everything built in this environment before you came.