Dancing Hoskuld wrote in another thread (*):
"I am trying to come up with a way to evolve your own religion and thus remove the current names religions from the game."
(*)
http://forums.civfanatics.com/showpost.php?p=13307149&postcount=73
I think that would be a very bad idea, as it would remove a lot of immersion from the game.
In its basics, a turn-based strategy game is about statistics and numbers.
Foodbuilding1 increases food production by 2 units while foodbuilding2 increases food production by another 2 units. You start with access to militaryunit1 which has 2 combat strength. Then you research weapontech2 which allows you to build militaryunit2 which has 3 combat strength etc.
While these stats and numbers are important, they are also bland and boring. The strength of the Civilization game series is its immersion in world history and real life. Instead of foodbuilding1,2 and 3 we have cheese makers, beer brewers and butcheries.
A band of cavemen with primitive spears attacking a dangerous wild mammoth sounds a lot more exciting than combatunit6 attacking animalunit12 with a 56% chance of success.
Movies about the Roman Empire have given glamour to the swordsman unit, while mounted knights and tanks have their own coolness from various movies.
I'd like to extend this to religions: everybody has ideas, opinions, about major world religions. From Christianity with its impressive cathedrals and witch burnings, to the bloodthirsty jihadists of Islam, to the meditating monks of Buddhism, to the public human sacrifices of the Aztecs, everybody has some imagery, and probably emotions (positive or negative) about the various world religions, and this adds a lot of spice to the game.
Throwing this out and replacing it by a do-it-yourself religious system where you can choose between religious direction A (+5% tech) or religious direction B (+5% combat strength of units) may perhaps deepen the strategic part of the game but would also be an immense loss of immersion.
Whatever you do, please keep the historical religions in the game for their immersion.