I'm working on a bronze-age fantasy setting based largely on ancient Egypt with a dash of Mesopotamia thrown in. I decided I didn't want the world to be too exact an analog for Earth, so I flipped a coin to determine the hemisphere (result: southern hemisphere) and rolled dice to determine the direction of the river flow (result: west to east). But otherwise, I've been using Egypt as my main inspiration for things like the physical environment and supportable population density, with a region resembling the Levant to the south, and some wooded mountains inspired by the
https://en.wikipedia.org/wiki/Zagros_Mountains added at a point roughly equivalent to the Nile's first cataract.
I'm concerned that this may not work hydrologically, though. If there's nothing but open ocean to the East, the climate should be more like the Yangtze river valley than the Nile river valley. I think I can solve that by having my setting's Big River empty into a relatively narrow sea with another large land mass not too far to the east, so I can justify the east coast desert. But I think I still might have a problem with the Big River's basin not getting enough rainfall to produce the Nile-like floods necessary to support the population density. I'm thinking of maybe flipping east and west across the entire setting—or is there a way I can avoid that? The idea of making the setting's physical environment subtly weird in ways that make perfect logical sense once you have all the information is something that appeals to me, assuming I can make it work.