Entirely up for discussion, especially since I'm short on ideas for some faction units. I just had a few brainstorms that would help set nations apart from each other, some for existing factions and others that we'll probably be getting in the future.
Militia armed with a smoothbore musket and a pike. Pikes were a rarity among militaries in this time period, but were used frequently by these Russian troops.
Lance-wielding cavalry. Their lances give them better forward melee reach at the cost of side-to-side melee capabilities.
On rare occasions, Austrian specialists made use of Girandoni repeating air rifles. They were not widely used due to the complicated logistics of keeping the rifles pressurized. In-game, air riflemen would have an unparalleled rate of fire but a very limited ammo pool to simulate the inconvenient logistics.
While all factions have access to an elite "Grenadier" assault class, Ottoman Janissary Grenadiers are unique due to their more literal interpretation of the title; they actually carry grenades. Grenades in-game would produce relatively small blasts and would almost never be able to kill full-health enemies, but they would be an exceptionally useful tool in softening up a stubborn enemy line.
Tribal militiamen of questionable sanity that excel with the use of swords, which is fortunate since it's the only weapon type they have besides their fists. Combines the speed bonus of Light Infantry with the HP bonus of Grenadiers.
Unique form of cavalry, striking a balance between the use of swords and carbines. Camels frighten horses and cannot be stolen by horse-riding cavalrymen.
Still need ideas for Britain, Prussia, and France...