One of the main problems with adding religions is the optimal saturation point, which Firaxis decided was 7 religions. I remember making this argument with Wonder additions too in some thread who's name I can't remember, but ultimately the message became this; "the more you add, the less unique everything becomes."
7 religions ensure that, given a standard build of 18-24 civs, there can be roughly 3 civs per religion (slightly less with 18 and slightly more with 24, with 21 being the point at which 7*3 becomes true) Adding many many more religions will create a case where everyone has their own unique religion, at which point as Syndrome from The Incredibles said "When everyone is super, no one will be." It will create an atmosphere of nothing but hate and warfare. Furthermore if there are less religions then that yoiu will get into a world of complete and total peace, with no one ever at war, or at least always the same people at war with the same enemies. Obviously both of these are extremes, and i've almost forgotten what I started writting this about. Essentially, I am Against adding new religions and giving the mod more than 7 religions (it could probably handle 8 pretty well if we want to add Zoroastrianism) because it could destabalize the entire thing.
About the religious schisms, if it can be modded, and I'm sure it can, perhaps civs should only get a diplomacy bonus if they share the same religion AND the same religious civic. Then the Religious civics could be considered to be, or perhaps remade into, different interpretations such as; fundamentalism, progressivism, or other ideas that I can't think of myself.
As for modding this, the schism feature I described should only require adding a simple AND statement to the AI sdk file, so that where normally it checks if 'civ A has same state religion as civ B' it now checks to see if 'civ A has same state religion AND religious civic as state B'.
