On thing that has bothered me, take for example the Kingdom of France. Every single lord of said kingdom wears a blue surcoat with fleur-de-lis. The same with every other faction, they all have a standardized outfit for the lords. This is historically incorrect. Each lord, hell every person of knightly class should have a distinct heraldic pattern.
Every single one should be different, the purpose of heraldry was identification. Think of it as a name tag.
That being said, people of the same household may have closely related heraldry. For example, say my heraldry was a large fleur-de-lis on a black field. My son's might consider of a large fleur-de-lis with two smaller ones on a black field.
This was done so that in the heat of battle they could identify one another, not only allies but enemies as well.
I think this variation is something that would greatly add to the game. Nothing could add more 'flavor' or a better look and feel.