Interesting idea.

Lets look at english Longbowmen as an example. Maybe you can train them to have different abilities

Make Wedge
Place stakes in the ground
Use different types of arrows
second quiver of arrows
Aim at different parts of the body or maybe at a mounted knights horse.
Flaming arrows
mounted/ unmounted
different types of contract levy/indenture
Different draws giving different ranges
Specialise - ie knight killers, castle attackers etc.....
dig potholes, ditches or use hedges
etc....

This is just ment to be an example of what could be done with specific units.