I've always been fascinated with the Mongols, and really a lot of the nomadic peoples of east Asia.
We know that the Jin Dynasty was considering changing its divide-and-rule approach to the Mongol tribes in the mid-12th century. Ultimately they stuck with the status quo, and the Mongols were never able to stop tearing each other apart in tribal wars and minor diasporas to get anything together, with later Chinese dynasties gradually vassalizing them and incorporating their lands. I always wondered what would've happened if the Mongols had managed to unify. Supposedly they had truly fearsome warriors and the ability to move vast self-supporting armies of elite cavalry over long distances. How far could they have parlayed that advantage if they could find someone to unite and lead them?
How successful do you think a unified Mongol country could have been in the Middle Ages, assuming a POD in the mid-to-late 12th century? What would the world look like?