I'd like to point out, during this time many crusades were sent to Jerusalum. Though maybe you should add Islam, Jewish, Buddisam and Christianity. So I have created a system called "The Pillars of 1257 AD Religion".
{Christanity based}
--------------------------------
-Maybe add a religion factor,
-Maybe add something like Christanity is a type of faction that goes inside factions, and Rome is the main capital, but it does not seek land and has few troops, but does have travelling priests, only troops are religious warriors like Templar Knights who fight only Islam and on Crusades.
-As a king you can build churches in cities, causing more Religion points towards a cristinaity. The more Churches the relation with the Pope, low amount of Churches and you are excommunicated.
-In cities if there is a church, you can now go to Churches like you go to taverns, where the Priest will be located, he'll like the wandering minstrels, execpt instead of romance, he'll talk about religion. Also he can help get you religious warroirs like Knights Templar if your high enough relation with the pope. You can also do religious quests to not only get higher relation with the pope but christanity as well.
-If high enough relation with a pope, you can join a crusade as a general, even if your not a lord. Also he can get you land if he likes you enough.
{Islam Based}
---------------------------
-Simmilar to Christianity, it's main City is Baghdad, and has a religous leader as well. It does most of the same stuff as Christanity, much is more of warriors and does not require churches or in their case temples to be in good terms with them, as they only have few temples, instead to have good relation you must instead do religious missions, which give you double relation if completed, and you get missions not at will but by messages you get very so often.
{Jewish Based}
--------------------
Capital City is Jerusalum, and is a more copy of Christanity, execpt they don't do crusades unless Jeruslum is taken over, they have no leader but a council of 3 priests, and they don't have any troops.
{Buddisam Based}
-------------------------
Really only the mongolians are buddists, unless you add china and asia and stuff, their capital city is the capital city of Mongolia, which I don't know if mongolia will be featured in the mod, and if Asia is featured, Hong Kong is the Capital. The like Islamic people, yet have no missions or crusades, and only way to be good with them is to be good with the High Buddist.