I have expressed before that having religious beliefs be exclusive doesn't make thematic or historical sense, but got told that would just lead to everyone picking the same, strongest beliefs.
Which... (1), yes, as you noted, many religions
do share many beliefs, so I consider that a feature, not a bug, and (2) if it is such a gameplay issue, then it seems like the obvious solution would be to just nerf the general purpose, "strongest" beliefs e.g. those dealing with Holy Sites in favor of more situational beliefs like Civ5's terrain or resource based beliefs.
It also kind of bothers me that if you're not the one to found a religion, you get 0 input on how it functions - not even within your own borders, not even if you're 99% of its population.
I saw someone bring up ecumenical councils before, so here's an idea - what if, one time per religion, you got to swap out one of a religion's beliefs for a different one, for the purposes of how that religion works within your borders, and the beliefs of the religion that your missionaries and pressure spread. This could include Founder beliefs, where you are now considered the founder of this new split off the religion, and the Holy City is the city where this reforming "ecumenical council" was held. This gives everyone
some control over how the religions in their borders work, but not as much control as if they had founded them themselves.
For example, imagine Arabia founds Islam with the beliefs (using Civ5's belief system as an example):
- Pantheon: Desert Folklore
- Founder: Pilgrimage
- Follower: Mosques
- Enhancer: Just War
- Reformation: Unity of the Prophets
When they spread this to Persia, Persia gets the opportunity to swap out one of these beliefs for a different one, to produce a new offshoot called, say, Persian Islam. Let's say:
- Pantheon: Desert Folklore
- Founder: Pilgrimage
- Follower: Mosques
- Enhancer: Religious Texts
- Reformation: Unity of the Prophets
Then let's say Persian Islam spreads to Turks, and they also get to swap out one belief to produce a new offshoot called Turkish Islam:
- Pantheon: God of the Open Sky
- Founder: Pilgrimage
- Follower: Mosques
- Enhancer: Religious Texts
- Reformation: Unity of the Prophets
And then let's say Turkish Islam spreads to the Caucasus to, say, the Circassians, who again get to swap out one belief to produce a new offshoot called Circassian Islam:
- Pantheon: God of the Open Sky
- Founder: Pilgrimage
- Follower: Mosques
- Enhancer: Religious Texts
- Reformation: Religious Fervor