from Wikipedia said:The College of Cardinals is the body of all cardinals of the Holy Roman Church.
A function of the college is to advise the pope about church matters when he summons them to an ordinary consistory.[1] It also convenes on the death or abdication of a pope as a papal conclave to elect a successor.[2] The college has no ruling power except during the sede vacante (papal vacancy) period, and even then its powers are extremely limited by the terms of the current law, which is laid down in the Apostolic constitution Universi Dominici Gregis and the Fundamental Law of Vatican City State.
Before 1555
From the 13th to 15th centuries, the size of the College of Cardinals never exceeded thirty, although there were more than thirty parishes and diaconal districts which could potentially have a titular holder; Pope John XXII (1316—1334) formalized this norm by limiting the College to twenty members.[2] In the ensuing century, increasing the size of the College became a method for the pope to raise funds for construction or war, cultivate European alliances, and dillute the strength of the College as a spiritual and political counterweight to papal supremacy
Do you think we should have this? I remember mentioning this before in the original Crusader King's forum's CK2 wish list thread, but I'm not sure if it was mentioned here yet. For CK2 you could create bishoprics which make Bishops, which in turn can be elected Cardinals, and as a Cardinal can vote for the next Pope, or even becoming a Pope himself.
Just like my earlier example of Royal Edicts there could be a Papal Edicts that may institute certain holy laws that could make things easier, or worse, depending. The only person with control of this would be the Pope (Player controlled if currently player's character is currently in that position). Aside from that, the Pope does the usual stuff such as approving crusades, excommunicating, etc.
Source #1: http://en.wikipedia.org/wiki/College_of_Cardinals
Source #2: http://en.wikipedia.org/wiki/Size_of_the_College_of_Cardinals