Not Holy, Not Roman, Not an Empire

Look at that dapper fellow, as if he would ever say anything misleading between all the lottery rigging he was doing

So, since playing in a Fantasy Medieval game is essentially a mish-mash of all the things that seem sort of cool about the Medieval period, I figured that I would set the beginning of my game in a pastiche of what was a mish-mash of a load of different things: The Holy Roman Empire. It is a great model to jam all the cool stuff that happens in Medieval Fantasy together: you want mercantile city states? They're there! You want proto-republics? You got it! You want Bishop Princes ruling ignorant serfs? Right over the next hill! You want a grand high king but don't want to worry about how complex that sort of rule would actually be? All-hail the Emperor! Politics, and trade, and warring states, and religious tensions all come together in the wonderful stew that was the Holy Roman Empire. There's a reason Games Workshop used it.

One reason this sort of model is so neat, is that you can have a war, without it dominating your whole story. You want your players to be able to wander around in a relatively politically homogeneous area, whilst having new types of culture just over the next hill. It works equally well to make a "points of light" trek through the frontiers of Empire, where Barons are only notionally under the auspices of a distant authority figure, and the wild forests remain untamed.

Using this you can flip back and forth between "Empire united against a common foe/great war against darkness" thing and "barons at each others throats" as the situation demands. There's always the possibility of setting a civil war not too far in the past, allowing for the "Wild West" style expansion at the fringes. Formerly glorious nobles exiled to the edges of the known world, far from the vengeance of court.

 The other thing that it saves on is having a common language. You can have all sorts of local languages and dialects, but it can be assumed that there is some form of Imperial lingua franca which covers the whole territory. It gives an in-universe reason for "common" to exist without it seeming too contrived. After all, there needs to be a way for all Imperial decrees and tax-codes to be distributed.