SJSerio
Centurion
My idea of returning religion would not match how they play in Civ4. I would disconnect religion from techs, except for some basic reliance (eg: obviously you can't found Hinduism if you only have Monotheism, and can't found Christianity if you don't have Monotheism). I would have certain techs increase the chance of a Great Prophet appearing and then the GP founds a religion.
That is a good idea. I think that beyond techs, other factors that could lead to the available religions to found could be resources available to the host Civ, as well as other religions. For example, you may need incense (a very important resource in many religions), and in order to found Christianity, you need Judaism within your borders (the holy city would be founded in (one of) the city(-ies) that has Judaism.
Also, I think that a civ shouldn't be able to adopt a religion until it is available in the capital city.