Ive posted this in suggestions Leoreth too, and will carry on any discussion there so not to distract from the SVN discussions! Thanks again though - and if you do think this might be interesting to implement, be happy to help with graphics/icons etc.
Shi'ism - Sunni schism
Shi'ism emerges in about 632AD following Muhammads death in 632.
The schism was over who should rule the Muslim Community.
Sunnis - Emerged from those factions, who sought an elected represenative chosen by chiefs and powerplayers among the Muslim community. This lead to a caste of political/theological leadership inspired by the 'companions' who accompanied Muhammad. They did not associate (Ali) the son of Muhammad (and the subject of Shia schism) as the rightful heir to their religious leadership, nor do they associate divine qualities to the descendents of Muhammad.
Shia - Emerged from those who saw direct descendents of Muhammad, whom they percieved as divine, (arguably akin to saints) and who believed the Muhammads son Ali as the rightful heir to their religion.
So the difference is both political and theological.
Historically the distinct differentiation of Shia and Sunnis does not significantly occur (in any geo-political context)
until the emergence of the first Shia states around 900. Note this is some 200+ years after the discovery of Islam.
During this period onto 1171 we have several major Shia states, these include the Fatimad Caliphate (Egypt/Most of North Africa) and the Buyid Dynasty.
It should be noted that one of the greatest contributions to the collapse of that huge Fatimad empire, was the abandonment of Shi'ism (and the failture to impose that upon them) in the face of resurgent Sunnism among the Berbers (comprising their main military cadres.)
http://en.wikipedia.org/wiki/History_of_Shi'a_Islam
So during early middle ages it was Shism that ruled dominant among the ruling classes of the greatest Muslim Empires (only to switch to Sunnism) before splitting once more back and forth between rival states and houses. This lead to inter-state conflict between Muslim states with of course denomination (being a contributing cause.)
No different really to the conflicts that arose between the Christian nations during the reformation periods.
Today that conflict still plays out, as anyone knows, Saudi Arabia (the bastion of Sunnism) is itself not a big fan of Iran... the bastian of Shi'ism.) With some 250 million Shia's in the world today... it seems a religion of enough contribution to conflict, culture and relevance today... to deserve inclusion ingame...
Gameplay Suggestions
The split between Sunni and Shia does not have to be instant. The political manifestation of the schism does not occur for some two hundred years after its foundation.
I would suggest for example
*event* Religious Change Event - Player accepts or rejects switching state religion to Shia, regardless of which Shi'ism will spread among some cities. (Shi'ism forms a significant minority in almost all Muslim countries even today.)
- Ideally this would see Islam becoming Sunnism, however I would argue (for simplicity) that islam could simply be renamed Sunnism, with Shia emerging later through the event. Obviously in a perfect world (of abusing Leoreths time) Islam could be replaced by Sunnism or Shia depending on the choice chosen.
Alternatives? This is the tough part? See the schism that occured here resulted in nations and ruling houses going to war - it was a geo-political event. Not something that can be reflected within the context of a unified abstract religion of 'Islam' ingame. So something that does not split Islam (and leads to nations sharing that religion falling foul of one anothers choices) may not properly reflect something that well... historically should!
For those of us in the West, with a european-centric perception of history and culture, the Sunni's and Shia's (and the reasoning for the schism) is I think difficult to understand. Especially if we force those shapes through the holes of our own understanding of Christian schism. At face value, its theological and political differences are abstract (and we lump all Muslims into well... one group.)
But I dont think we should or can, without ignoring one of the greatest untold conflicts that arose between the peoples of Noth Africa and the Middle East, with repercussions we feel right now... *hint Saudi Arabia vs Iran*