I think it will be important to have an actual religious split to fit into the vanilla portrayal in CK2, as well as to not have later Sunni caliphs portrayed as religious heads for all Muslims once the faith starts splintering.It may be simpler to avoid having to split Islam and do something like what CK2 plus does with Greek/Latin rite traits as a way to split up the faithful.I didn't see any discussion about how islamic splits into shiite and sunni. I know there is a rather buggy event in the wtwsms events file, did you expand on that or revamp it entirely?
Depending on who votes for Ali in the Shura decides who the initial Sunnis and Shias are.
Right now though, I don't have much included. I've put in a trait shiat_ali that a few characters start with who are warmer to the Alids/view them as rightful caliphs. However, I still need to expand so that can trait can propogate and then later be used to tag who should turn Shiite.
I'm thinking for simplicity (/localisation reasons) that Sunni and Shiite will stay with our definitions so if say Islamic Caliph is an Alid then they'll eventually turn Shiite.