1. Rome should be Orthodox from 500AD if we are going to be honest. Until the 800s the Pope in Rome was essentially a Greek speaking Byzantine puppet.
2. Aghlabids from Tunis launched the invasion of Sicily, not just random "Muslims". I think a Tunis civilization is in the works? In my opinion an unplayable Aghlabids would make most sense, and have the AI try and invade all of Southern Italy. On the barbarian side however, Muslim pirate ships would be nice, and on the reverse Christian (Italian - Pisa, Genoa, Venice specifically) pirate ships would also be nice.
3. The Normans came as roving bands of mercenaries first, not conquerors. If they could be represented as mercenaries and/or even barbarians that would be neat. The de Hauteville's eventually started to seize territory (staring north of Naples), but they were extremely fragmented. A unitary Norman state in Southern Italy only begins with Roger II (1130) who proclaimed himself King. But the Byzantines weren't alone, there was also Lombard princes, independent Greek states, and of course the Muslim emirates. Until 1130 the Normans would best be depicted as independent/barbarian armies, and then in ~1100 spawn in Sicily, and have to unite Southern Italy. But given how fragile and transitory the state was, it is still bizarre why they are a fully playable civ.
4. Having a Byzantine goal to defend your Italian possessions would be neat; especially since the game begins in 500.