I think brigands would be relatively easy to implement sensibly and all the basic triggers are already there in the program somewhere.

For example:

Mercenaries: Take the number of unemployed mercenary companies available in a given region and take a percentage of this number to produce a random group of brigands armed in accordance with one or more or the mercenary units which remain unhired.

Unemployed Soldiers: Every time a military unit is disbanded in a region add a percentage of that unit to the number of brigands in that region and arm them in accordance with the disbanded unit.

Deserters and dispossessed soldiers Take a percentage of all soldiers who survive a battle where the defeated army disperse and add them to the tally of local brigands. You might also want to add them to the number of available mercenaries too.

Brigand Behavior
Brigands should not behave like rebels, they have no axe to grind, they are merely trying to survive. Thus they should not attack anyone, or anything, instead they should interdict trade a bit like land-based pirates and avoid direct conflict whenever possible.

By comparison Revolts whether peasant or noble-led should have a clear target agenda and behave aggressively in order to achieve their goal. They should also gather strength over time if not dealt with quickly.