Here's another proposal, for a minor map change.
I've been researching the period of the Crusades, and came across several references to Byzantine silk that I didn't understand. On looking it up, I discovered that around the year 550 AD, itinerant monks discovered in China how silk was produced, and later smuggled silkworm eggs or larvae out of Central Asia to be brought to be brought to Constantinople. The resulting silk production was in fact a critical part of the Byzantine economy from that time until the fall of Constantinople almost 1000 years later.
The Byzantine monopoly was ended during the Second Crusade around 1150 AD, when silk weavers were captured and brought to Sicily, spreading silk production and trade further into Western Europe. The same occurred during the Fourth Crusade (around 1200 AD) when Constantinople was sacked and the silk factories were brought to Venice and other cities in Italy.
All that is context for my suggested proposal:
1) If silk isn't a luxury resource in RFCE, it should be added.
2) There should be a silk resource placed near Constantinople, preferably in the province of Thrace (on the European side of the Bosporus strait).
3) Alternately, if RFCE allows resources to be added to the map after the game starts, then the silk in Thrace should appear around 550 AD (or as close to 554 AD as the turn clock will allow). Likewise, to reflect the spread of silk production to Italy, another silk resource should also appear in Sicily around 1147 AD, and potentially near Florence (on the west side of the Apennines, as close to the city of Lucca as the map will allow) around 1204 AD.
4) Alternately, instead of manually adding silk resources to these tiles, you could create a new 'Project' that becomes available very early on that provides on completion one or two silk resources. The prerequisite for this project should be "Own a city in the Byzantine core" -- which would allow the Byzantines to start working on the project immediately and complete it at the proper historical time, as well as enable the Italian states to work on the project as long as they conquer a Byzantine city (per the historical method). The project should have 2-3 slots, like some of the later colonial projects.
Finally, this is slightly unrelated, but I'd really encourage there to be another project or wonder or bonus for the Byzantine Empire related to the Silk Road. Considering how vital that was for the Byzantine economy, it really feels ahistorical for that not to be reflected in the game. You could even make the 'Silk Road' into multiple projects, that expire periodically and must be rebuilt. The prerequisite for building the project would be owning a tile on the far eastern edge of the game map, so it would be impossible to rebuild if the eastern part of the Byzantine Empire was overrun. The Arabian civ would still be able to build it, which should make it easier to play and win as that civ.