Each faction has a religious sub-faction which is not fully controllable by the player. Say, for every 3 provinces owned, 1 is a religious province.
In order to use this province to create units and expand buildings you must keep the religious leader happy (in a similar way to medieval 2 papal system). If you dont keep the religious leader happy you can have various complications like refusal to let the player create units in religious provinces, to religious units currently in the players army deserting, to civil war between church and state.
Also the player would have to pay tithes to the church each turn. The happier the religious leader is the less the tithes would cost.
As the player becomes more powerful (in terms of provinces conquered), so too does the church become more powerful and more demanding. So the player has to balance their expansion with catering to the church.
I think a system similar to this would help prevent steamrollering in later gameplay as the player cant just build and conquer once they become very powerful, but must pay money or perform inconvenient missions to keep the church happy, or pay the (probably dire) consequences.
Also i would like more randomness in each campaign played. ie each faction starts the game with different levels of aggressiveness, diplomacy aptitude, trading aptitude etc. and each time you start a new campaign, the computer randomly assigns various traits to each faction. this way you wont know for sure what your up against each time you start a new campaign.
keep up the good work guys. great games.