Great idea for the mod! Ive always loved that period and ive always sympathised the Byzantines...they were the bulkwark of Christendom but they were engaged in constant rivalry with the West which only weakened them =/
About the xenophobia and anti-semitism, i agree it happened but in that regard, youd have to decide whether this mod would be a darker mod with xenophobia, anti-semitism and etc or ignore it. I would be happy with either, maybe try and script pogroms against Jews and maybe the peasant crusaders every once in a while?
One awesome idea/feature, which would tie in pretty nicely, would be money lending. Many kings would often need to borrow money to pay for their campaigns and Jews were kind of well off in that they were allowed by their religion to engage in usury or charging interest on loans. As a result, this added to the hostility towards the Jews and it would add an interesting dimension, you can decide whether to borrow money from them or join in the pogroms and rob/steal!
Also, another cool dimension to be added in would be the rivalry between the Italian merchant cities, most notable of which were Genoa, Pisa and Venice. No Crusading expedition could be successful without the sea power and also the transport ships afforded by these cities and the Outremer also relied on them for trade and income and as such, they were given VAST concessions in commerce and each had their own enclave in the big cities. However, they were intensely jealous of each other which led to many a conflict between Genoa, Pisa and Venice, each trying to vie for supremacy at the expense of the others. On various occasions, open fighting and naval warfare emerged between these cities to the detriment of the Outremer They also engaged in trade and intrigue with the Egyptians most notably as well as the Caliphs of Damascus and their subjects. You could say that they couldnt really look beyond their own coffers and purses which is a very sad thing indeed.
The rivalry between the Knightly orders would also be pretty neat.
I have a lot more ideas but i think id need more time to make them coherent XD Im an avid student of history, like i live on this stuff and its what im studying at uni as well so ill try to come up with more ideas and advice and if you want, i could offer some historical info and etc should you wish it!
Im hopeless at coding and modding though! Sorry! >.<