Jerzy, I can not rmember the Byzantine Masters but I guess the problem would be to know for what period are they designed and if the corresponding enemy already exists or not.
As to Medieval periods I have just remembered that there is 1 well known period that has not been covered and could get some support, The War of the Roses. A number of known characters: Warwick "The King Maker", Richard the III. Practically the last hurrah for the English Longbow being the most formidable weapon.
So what do any of you Medieval Enthusiasts think of a WotR line from Hat?