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The idea isn't bad, but introducing real-world nations into "Heroes of Might and Magic" doesn't seem like a good fit, in my opinion. They already tried introducing a Slavic race, and it didn't work. This is because it's a fantasy world, and the nations should be fantastical. You could create a race whose culture is similar to the Mongolian one. For example, a race of centaurs. I know they originate from Greece, but elves also originated in England (they did, at least), and centaur customs can easily be compared to Mongolian ones. A nomadic lifestyle, an emphasis on ranged units, and so on. I could even help with unit names. The main thing is to use your imagination, and who knows, maybe they could become a great replacement for the Barbarians!