Let's say there's a big human-centric (though far from human-only) High Medieval mod being made. Let's say there's a bit of a religious system in it. And let's say we're populating the tropical regions, and that there will be pseudo-Byzantines regardless of anything else. And let's say there are these two particular civilizations that could each easily fit into one of two concepts.
One is sort of roughly like pre-Islamic Persians, has a caste system and a religion based around fire, water, and light, and their religion involves large stone fire- and water-shrines at which they offer sacrifices of meat, fat, milk, water, and wine, use sacred ash for blessings and exorcisms, and invoke elemental spirits.
Then you've got these sort of pseudo-Arabs, with a fairly simplistic, minimalist religion based around purity, praying on prayer rugs, and binding, enslaving, and exorcising magical and elemental spirits.
So, there are two set-ups I could see working. In the one, the pseudo-Persians are part of a mighty, caste-based, theocratic, roughly Sassanid empire, while the pseudo-Arabs live in small, vaguely pre-Islamic desert kingdoms and city-states.
In the other, the pseudo-Arabs conquered the pseudo-Persians, binding/enslaving/exorcising the latter's elemental allies with their magic, and live in large, sophisticated, 1001 Nights-esque high medieval sultanates, while the pseudo-Persians now live small, sort of vaguely Kurdish tribes in the desert and mountains, driven away by the pseudo-Arabs.
Which would you see working best? From a game-play and narrative perspective. The latter would definitely lend a more medieval feel, but at the same time the former might work better with how complex I have the religious system of the pseudo-Persians.
EDIT: Also, both versions of each are basically fully finished, I just need to decide which versions should go in to the release... version.