America was by no means the only destination of the slave trade.
I don't think that the Statue of Zeus should have that ability - it just isn't useful enough in this map, seeing as there probably won't be many barbarians left by the time it is built. I vote to remove the Statue of Zeus and put the Oracle in it's place.
Stonehenge isn't a very good wonder; it was built around 3000 BC by a small tribe of people that crossed the channel from Normandy and never had any significance outside of England during the time period it was built, and later it grew into only an intriguing ruin.
I say that the Kabbah (Black Stone of Mecca) should be like the Colossus and Sacred Ganges; the Islamic requirement that every muslim must visit the city of Mecca generated boatloads of trade.
Possible Christian wonders:
- Holy Roman Empire (Get the Pope to name your country the Holy Roman Empire, no clue for effect)
- Justinian's Code (Basis of all Western law, maybe add Courthouses to every city)
- City of God (Book written by St. Augustine, pushed that every city needs a piece of the divine (A church) to guide them, effect should be obvious)
Could you rename the Knights Templar to the Crusades? It seems that the game had a mix up of cause and effect; the Crusades led to Crusaders establishing the Crusader Kingdoms in the holy lands, and then the Knights Templar was established to protect the Crusader Kingdoms.
I also would like to push for the distinction between early monarchies, which were little more than despotisms with religious approval, and the later feudal monarchies. The difference between the monarchy of Babylon and the monarchy of England is astounding.
Additionally, if you feel that the Christian countries still need more of a boost due to their sheer number, you could add the Romanesque churches as a step between temples and cathedrals. It would help every Christian country, and would certainly be realistic. I'd have them be like the temple, just available later.
Samurai fought in two real conflicts: the first Mongol invasion (Which did hit land) and that huge civil war. Both were largely defensive battles - one was an invasion, the other was a stalemate until arquebusiers came along. Therefore, it might make sense for the Samurai to be a primarily defensive unit. Maybe. It's really a design decision, though.
Finally, when do you plan on making these wonders obsolete? Some, like the Statue of Liberty, really need to go obsolete after a while. (Which reminds me, if the Statue of Liberty requires Mustangs, then will you move the Mustang resource back from Military Tradition a little bit? Democracy comes before Mil. Trad. if my memory serves me right.)