I believe that banner carriers should be a unit you can purchase from new towns called "Bannlets". They should require linen and hardwood instead of denars to upgrade, and have a buff that removes all mounts within a 10m radius. They should have almost no armor but be completely immune to arrow and other missile fire. If one dies it should inspire every unit within 20m to instantly kill anything they touch
(obviously restricted to 2 colors and a single icon for banner for bearers, for realism)