Religion

Religions develop much the same as tech. As such, conversion happens at the level of population not provinces. Foreign priests and populations can influence other locals and gain converts. Additionally, populations will begin developing heretical ideas(fallibility of the religious leadership, division of religion and government, etc) to challenge established doctrine then gather followers and grow their political power to push their agenda on the country. The player will have the ability to influence the population, regulate religions, crack down on heretical beliefs, etc.