For some reason heavy armor is actually not as restrictive on the body in warband. Its not realistic and needs to be looked at. It slows down your movement, but not by much and you still attack the same speed as if you are wearing light armor. So my suggestion is to make it so when you are wearing heavy armor, you walk slower, you attack slower, and you have decreased stamania (if its in the game). This is so we can actually see light infantry have uses besides just being worser troops. This could make the game even more fun when you have to find a balance between light and heavy units.