It would be amazing if routiers could be worked into the game at some later date.
Basically, routiers were soldiers, usually in English service (though they were of varied nationality: Gascon, Breton, Brabantine, German) who had turned brigand when peace broke out. But they weren't roaming bands of wild men: they were highly organized military companies, usually consisting of about 4:1 mounted infantry to men-at-arms. In many cases, they were garrison troops in captured castles who refused to relinquish them to the French authorities. They formed themselves into companies, ravaged the countryside, extorted money from and/or captured towns, took weakly defended castles, and basically destroyed huge swathes of France, especially in Burgundy, the Limousin, and the Mediterranean coast. The royal government viewed them as a local problem, to be dealt with by those most effected. Bribery seems to have been the most common method of getting them to relinquish their strongholds. Fighting was chancier, as the largest band, a conglomerate called the Great Company, mustered as many as 4,000 trained soldiers.