The GodMOD
So. Ive been thinking about how to improve on religion in Civ4. My first big thought is that we really need to increase the number of religions available, not in total number then in potential. First off I find it annoying and jarring to have my French civilization discover Hinduism, or my Aztecs Judaism. It gives religion a generic, totally acultural feeling. Furthermore, its not much fun to know that if you shoot for an early religion youre always going to be one of a couple religions. How many times do you really want to be Hindu, anyway?
Secondarily, while many applaud Firaxis for making religion very vague and inoffensive, and I understand why they did it that way, Id like to see it a little more realistic. Freedom of religion gives you a bonus for having lots of religions, but Id like to see a penalty for having foreign religions, especially in the early game when state religions are big. I want to be able to persecute heretics and drive religions out of my country. Furthermore, it would be fun to have some for of trait system.
Therefore, Id like to make two different mods. GodMOD Lite, which makes no rules changes, and just increases the number of religious labels/names, and GodMOD Full which introduces some new concepts. I plan on plugging away at this myself, but I dont really know much scripting yet so it may take some time. If anyone else likes the idea Id be happy to have some help!

I envision GodMOD Lite as working thusly: The civilizations are split into cultural groups. Say, Asian, European, MidEast/Africa, and American. Each group gets a different religion when they discover the same techs. So if Asian civ discovers Polytheism, they get Hinduism, if a European civ discovers it they might get Norse religion or Hellenistic (Zeus and pals). Youd still only get seven religions in any one game, but theyd be different religions each game. Youd also lose the religions being quite so tied to when they appeared historically but that barely works as is.
The big challenge for GodMOD Lite is probably tweaking the UI in the religion and city screen. Before any religion is discovered each icon should probably just be a big question mark and then be replace by the appropriate icon when founded.
I dont envision much graphics needs beyond icons for the religions. The 3D temples and such can stay exactly the same, and even the temple icons can just be tweaked a bit with the new religious symbol. Similarly, to make things easier the missionary graphics can remain pretty much the same, maybe tweaking a skin or two to give different color robes or something.
Heres a very, VERY rough list of religions and the techs theyre tied to that I drew up.
Religions:
Asian
1.) Hinduism (Polythe)
2.) Buddhism (Med)
3.) Shiite Islam (Mono)
4.) Shinto (Theology)
5.) Taoist (Code Laws)
6.) Confucisism (Philo)
7.) ??? (Divine Right)
European:
1.) Norse (Polytheisic)
2.) Druidic (Medi)
3.) Catholic (Mono)
4.) Orthodox (Theology)
5.) Protestantism (Code Laws)
6.) ??? (Philo)
7.) Mormon (Divine Right)
Euro Unique: Greeks and Romans get Hellenistic instead of Norse.
American:
1.) Totemic (Poly)
2.) Shamanistic (Med)
3.) Great Father (Mono)
4.) Spirit Dance (Theology)
5.) Olmec (Code Laws)
6.) ??? (Philo)
7.) Mormon (Divine Right)
Mid-East
1.) Osirian/Egyptian (Poly)
2.) Zorarostarism (Med)
3.) Jewish (Mono)
4.) Sunni Islam (Theo)
5.) Shiite Islam (Code Laws)
6.) Coptic Christianity (Philo)
7.) ??? (Divine Right)
Asian: China, India, Japan, Mongolia
Europe: England, France, Germany, Greece, Rome, Russia, Spain
America: America, Aztecs, Inca
Mid East: Arabs, Egypt, Mali, Persia
As for GodMod Full, I think it would be a good idea to get the Lite version up and working first, but I envision a couple things. First off, if you have a state religion then any foreign religion in a city then that causes one unhappy face. So clearly if you want to take the route of tolerance, you could build that religion a temple giving you a net zero effect from that religion. Or you could send a cheaper missionary to persecute the religion and remove it from the city. Probably you could only persecute in your own territory, not other peoples.
If we did this we want to reduce the cost of missionaries slightly, and probably increase the total number you can have slightly as they would be getting more internal uses. Alternately we could make a fairly cheap Inquisitor unit that cant convert and can only persecute.
The second thought I had would be to allow a trait system. Obviously there are way too many religions to make traits for each, so I figured we could have a trait pool of ~10 traits, and when you discovered a religion you got to pick a trait. So in one game Catholics might be Evangelical, and in another they might be Fanatic or have strict Dietary Laws. Some traits might not unlock until certain techs, so if you want to be Evangelical or Fanatic you might not want to jump for one of the first religions available as they have a smaller total trait pool. Here are some possible traits I dreamt up.
Artistic: Temples provide double culture points.
Dietary Laws: Temples provide a +1 bonus to health
Evangelical: Free missionary on founding religion and half cost missionaries. Available with Theocracy.
Fanatic: Half cost Inquisitors. Cathedrals provide one free XP point for all units built in their city.
Inspired: Free Great Prophet and two free missionaries. Probably available last, with Divine Right.
Monastic: Temples provide a +10% to research.
Tolerant: Non-state religions do not cause unhappiness.