That's not a bad idea. How about something like this:
Let priests have significant influence over the peasants who believe in their faith? At low levels, they could nudge morale and loyalty up and down. At higher levels, it might be reasonable to give them the ability to raise up a peasant mob to kick Lords or knights out of their estates, or even (if the priest's skill is high enough, and there's not enough troops in the region to stop it) claim the region as a new outpost of the religion (naturally, giving the priest the lordship in the process!).
...Hey, wait a second....
Those types of priest powers are good for quelling internal threats. They can be used to destroy dissenters, such as a Duke who threatens secession, by instantly ousting him from power, so that a more conservative character can take his place.
But as for dealing with external threats, they are useless. Hostile realms can simply arrest you before you start preaching, denying your religious spread.
Thus those powers do little to encourage intrigue or warfare, as they actively work to harm those who might initiate such things, while being incapable of initiating those things itself. Religion is a tool for preserving status quo, nothing more.
