Here's what I'd love to see in FfH, better system of religion. As it is a city is either from a religion or not, and can be from several religions.
What I would love to see is to actually have religions weighted so that the players would know how religion is affecting them, exactly.
Ie. Innsmouth have a population of 8, 5 of them follow OO, 2 follows RoK, 1 doesn't follow any particular religion.
Religion spread per population point, so it can expand "naturally" in the same city, or spread to a different city.
Religion %age for Religion victory can also be read in raw number of population points (ie. You Founded The Order, 25 pop points out of 250 world pop point follow order, you got 10%)
Religious modifiers are weighted to actual spread of religion, not just state religion. Ie. If you have 10 pop points following AV and follow the Order as state religion with 20 pop points following the Order, Basium will you less than if you had no followers of the AV (Which therefore makes mass murders act as diplomatic tools, but it's a dark fantasy mod after all...).
Civilizations using Theocracy, Religion, Religious Discipline will get a +100% religion diplomatic modifier (after all, you DO expect them to not not treat religion lightly if they adopted all of these civics)
Disciple Units: (Adept, Priest, High Priest, Patriarch)
Instead of converting a city, they can preach their religion to convert x population point of non-followers into followers of their religion, where x is one for adepts, 2 for priests, 3 for high priests, 4 for patriarch
There is a chance the unit will die in the process, 60% for adepts, 40% for priests, 20% for high priests, none for patriarch.
Additionally, a new Promotion would be available, that would allow to remove x point of population following a different religion - therefore removing religion gradually, but also killing population in the process.
There is a chance the unit will die in the process, 100% for adepts, 80% for priests, 40% for high priests, 20% for patriarch.
Population killed in the process will create Manes and Angels if from AV or Order.
Slavery would prompt for which citizens you would like to sacrifice (no followers, religion A, etc.).
Alternatively, Slavery would be remodeled, removing the option of whipping people to get productions. Instead, you have the option to draft/build workers or use existing workers to work an additional tile's food OR production (never gold). The tile would be unworkable by a pop point or a second worker.
The interface would prompt for which city to carry the goods to. Along with the output from the worker, the city gets an additional +1 unhappiness and -1 gold (not commerce).
Inquisitor would be unchanged, as it converts people rather than just exterminate them.
And a stuff I would _looove_ to see, schisms !
If you send a Great Prophet in the Holy city of a religion you have the option to start a schism. The said religion will then be cut in two, one named "The True" and the other named "The Real" (so that you can have a holy war between the True Orders from Heaven and the Real Orders of Heaven).

(I prefer the names True and Real to New and Old, or Reformed, etc. because I think it shows more fanaticism and fits FfH in a better way...)
Between 30 and 50% of the religion followers population points will become True, while 50 to 70% will become Real.
Religious buildings will become of the religion with the most followers in the city.
Units with Religion tags all belong to the larger religion of the two - this is because they have not changed religion, it is the religion that has changed.
Holy Shrines consider both religions to be the religion they are the shrines for.
Thats my 2 cents
