Everything, units, cities, wonders of the ancient world, battle formations and heaps of other things.....
I've worked on more than one mod tho, I used to help with Rome: Total Realism, Europa Barbarorum and now I'm working part time to help Chivalry: Total War and Invasio Barbarorum.
Invasio Barbarorum is based around the fall of the Roman Empire, the Rise of the new second Persian Empire and massive hordes, Huns, Franks, Vandals, Ostrogoths, Visigoths, Slavs and a slew of other factions. Chivalry is based on the original Medieval Total War, set at the end of the 11th century. The Byzantines have lost most of Anatolia to the Seljuk goat lovers, the Normans have control of Sicily, the Moors are in Spain and the 1st Crusade is about to start... If you still play Rome: Total War, I'd reccomend any of the above modifications...