My own guesses so far are:
Ariselle--I think it's actually England. It's army has longbowmen, dragoons, and knights and it's an island nation with a powerful navy.
Boruvian Empire--backstory is pretty strongly Byzantine/Eastern Roman Empire, though it uses hussars as it's cavalry so it's a little of a mix. It also has unused units called cataphracts and hospitallers to further confuse things.
Myrmont--France. It's got gendarmes, one of the game's few very heavy cavalry that isn't an Elite unit and was the heavy cavalry used in late Medieval/Renaissance France. It also has a heavily fortified border with Hadrigel, which seems to be Germany, though allusion to the maginot line may be unintentional.
Hadrigel--Germany/Holy Roman Empire. Ritter was the German word for knight, and landsknechts were famous late-Medieval German mercenaries.
Jotland--fairly clearly based off viking-era Norse countries.
Vessoi--I initially thought they were Mongols, but their units make me think more of the common portrayal of European barbarian tribes, with berserkers and horde warriors and mastadons for a fantasy touch.
Valegorn Palatinate--the Papal States.
Ladvia--Dracula-inspired Wallachia/Romania.
I think Elidon is based off of Italian states, though I'm far from certain. Elidon has a strong focus on mercenaries, which was a common issue with Medieval Italian states, and a good navy. Averine may also have a similar inspiration but I am not certain.
Of course many of the non-human factions seem to have inspiration from common fantasy portrayals. One I find particularly interesting is that the Irony Barony uses the "weak but numerous" orcs similar to Tolkien's, while Khazoth's "fewer but strong and brave orcs" it another common fantasy portrayal of orcs used in other works.
