Perhaps allowing players to produce 'veteran' and normal types of some or all units (maybe on a building basis?)('veteran' peasants might be considered silly, eg., the whole point about these guys is that don't fight regularly; perhaps they could be changed to 'looting knife mob' or 'jacquerie' or something)

So, normal archers would have less ability to penetrate armour cos they weren't experienced enough to make sure they used the right arrows or were less well-equipped than the veterans. This would not be the same as higher valour, but more based on fighting wars fairly frequently.