Making sure that every faction has a light infantry, and a skirmisher unit.
Britain:
Light: 51st
Skirmishers: 95 Rifles
France:
Light: Some Voltigeur unit
Skirmishers: Some Chasseur unit
Austria:
Light Infantry: Grenzers
Skirmishers: Jeager regiment
Prussia
Light Infantry: Fusiliers (My suggestion would be No.6 West Prussian Infantry regiment, 3rd battalion)
Skirmishers: Silesian Jeager regiment
Russia
Light Infantry: Some sort of light infantry
Skirmishers: 21y jeagers
Other Suggestions:
- Give all units a sergeant class, with a musket.
- Improve bracing
Realistically FSE, you could VERY VERY easily put a Prussian fusilier unit in the game. They would have the same uniform as the 8th Brandenburgish, but with black crossbelts, and different facing colors.