I like the idea of religion being founded by a Great Prophet instead of some tech. One interesting thing would be the ability to found religions in City-States - instead of founding the religion in the player's own civ, a player could send their Great Prophet to a friendly or allied CS and found a religion there. If a particular CS is the holy city of religion A, all civs that have religion A as a state religion could have their influence with the city-state degrade more slowly. When the player founds a religion in one of their own cities, the player can then choose between different Holy Temples (or whatever) that are mutually exclusive; build one, and the others are no longer available. Each offers bonuses based on the spread of their religion, but the bonuses differ: you could have a Culture-producing temple, a Gold-producing temple, or a Happiness-producing temple. These wouldn't be +1 per city with the religion, necessarily; it might be +0.3 per city, or whatever, to keep it balanced. Every Holy City you own beyond 1, however, could incur major Unhappiness, to simulate internal friction. I actually liked missionaries - random spread is kind of boring and uninvolved; missionaries add an extra mechanic to actively play around with in peacetime. All cities directly linked to a city with a religion would then be subject to a slow, random spread of the religion. Consumable missionaries kind of make sense (you could imagine them each founding a temple in the target city, and staying there to teach), but it might be better to have them permanent; they get in a city, and every 8 turns (or something) they convert one citizen from that city to the religion in question, preferably of the "irreligious" citizens, though if all citizens have a religion and the missionary's religion isn't represented, it could convert one of the citizens to a new religion (only one). I can't remember if they were in Civ4, but missionaries should be attackable and destroyable. Religion, then, could be a function of the citizens - say, in a city with 12 citizens, 8 might be Buddhist and 4 might be Jewish. A city with multiple religions could incur Unhappiness; for example, -1 for 2-3 religions, -2 for 4-6 religions. A city with a single religion, though, might get incur +1 Culture (an encouragement to stop the random spread of religion using something similar to Theocracy from Civ4). Religion shouldn't have as big an effect on diplomacy. Somebody mentionned it easing border tensions, and I think this would be a good compromise: civs which share borders and a religion do not suffer diplomatic penalties due to border closeness. CS which share a religion with a civ will not degrade their influence as quickly (say 10% less quickly). It might also be interesting to implement persecution; for the price of a major diplomacy hit and temporary unhappiness the player could activate a random elimination of 1 citizen-religion per turn (or per 2 turns) of either a target religion, or any non-state religion, thus reducing unhappiness and internal tensions. Additionally, when capturing a second Holy City, there ought to be the option to Raze the Holy Temple, thus avoiding the Unhappiness and losing out on the bonus (it can't be built anymore).