My Greek campaign still going great. I can't get enough of this game. As a huge fan of Byzantium I have a few things to offer. I'll start on the lighter side.
I found this finnish metal band who sings a pretty sweet song about the Varangian Guard. Worth a listen. I'm adding it to the civ 4 soundtrack
http://www.youtube.com/watch?v=1x-5ZkTMyMc
Now on a more serious note. After the debate with the Greek Water wheel, which doesn't make any sense for a variety of reasons, I brainstormed an idea for a significantly more ideal improvement. Forts. Asia minor is littered with them, and the Greeks absolutely depended on them for significant periods, especially throughout the byzantine era. Forts in many ways defined both the rise and the fall of the late greeks, and are worth receiving some attention in the game.
Now as for specifics, that's trickier. There's a lot of routes to take, and I don't know what the limits are for defensive improvements. Maybe an additional defensive bonus, a faster construction time, or an additional +heal bonus. i'm trying to research more for some better ideas, but researching the byzantine empire is a pain.
Along these lines. The Great walls of Constantinople should be strongly considered as a great wonder. The influence these walls had were far greater then the majority of wonders currently in existence, and their effectiveness was unmatched. At the very least these walls should provide a bonus to great engineers, a large cultural bonus, and a large defensive bonus. The walls are simply too significant to be left out of this mod.
Heres a little video on the walls, though this left out some other important aspects of the wall, such as the great chain.
http://ed.ted.com/lessons/the-city-of-walls-constantinople-lars-brownworth
EDIT: @Tach
The culture from the great works themselves will double over time just like most culture buildings, because of this they can't really be further buffed since this already makes them very strong compared to the flat +4 gold +12 culture gained then the specialist joins the city.