Normans for sure, stuff the sicilians(as a faction), they would be a nice factor in the equation that is medieval total war. In real life they were a handful for the Pope, the English, the Byzantines, the Germans and the Almohads. I reckon they would be a very very nasty faction to include in an expansion pack, they start with provinces like Normandy(From the English), Sicily and Naples(from the Byzantines)as an enemy they would be interesting to confront, as a faction a strategic starting posiiton in terms of conquest, etc.
Unique Units I would like to see is the Norman Knight.
Robert Guiscard as the starting leader of the faction and glorious achievements like a Crusade to Antioch or something like that.