A few suggestion I had for the vanilla game but I think most still apply
1. Iqta members are more likely to gain the "Ambitious" trait
2. Decadence causes negative opinion in your vassals -5 per 10% this replaces the invasion
3. Vassal AI will be more likely to aspire to having a kingdom title
4. Vassal reaction to raised crown authority is greater as is willingness to lower it (how much greater is debatable)
5. Vassals outside de jure territory might ask to become a tributary instead of a vassal and if reject will be disappointed (I don't have horse lords so this might not be something that makes all that much sense it just sounded like a good idea)
Caliphate
5. Powerful vassals in de jure territory might aspire to usurp the caliph title (different requrments for different caliphates for instance Sunnis by piety and Shias by Sayyid or Mirza trait)
6. When a Muslim with the Caliph as head usurps or creates a kingdom the caliph may either legitimize it or reject it. Legitimizing it leads to a positive opinion from the creator of the kingdom while rejecting it leads to mutual negative opinions if the king is a vassal the caliph is likely to strip him of the title and if he is independent the caliph can give the right to invading the kingdom to a vassal of his choosing or be requested for it (reflecting things like the Samanid invasion of the Saffarids at the urging of the Abbasid caliph) also part of it should be dependent on proximity and importance of the title with regards to legitimacy I don't think the caliph gives a shit of a Muslim claims the kingdom of Perm for instance
7. There can be two Caliphs maximum at any given time (per sect), Sunni muslims ca declare themselves caliphs occupying only Cordoba
8. Independent Shias and Ibadis can proclaim themselves Imams with either sufficient piety or the right blood line (Sayyid, Mizra) giving themselves a +1 Piety/Month and +5 Opinion (Title hierarchy: Caliph>Imam>Secular title)
9. Druze, Sufi (Harufi), Yazidi, Zikri should be separate branches and the Jurisprudences should instead represent heresies (I.E Hanafi, Hanbali, Malik) and they should confer a -10 religious differences between each other.
10. Pagan converts gain folk islam as opposed to any specific form (Unless the are neighbors to a settled realm that has a specific form) folk islam do not get any of the decisions associated with common islam aside from Hajj nor become neither Imams or Caliphs upon settling, feudalizing or becoming a republic the choice of Sect and Jurisprudence appeares as an event to the player.
11. Muslims cannot declare Jihad (as a form of crusade) standard holy war is renamed Jihad
12. Depose Rival Caliph is a Casus Belli that is against all other Caliphs (Irrespective of sect) upon winning your opponent loses the Title of Caliph and this causes a split that causes all vassals outside of the losers de jure to become independent all however become the winners tributes and the loser cannot take the title caliphs until the next successor takes the throne.
13. If you have the Caliph as a Vassal you can scheme to revoke the title and take it for yourself given certain condition this will also cause vassals to have a very negative opinion of you and confer decadence
All Religions
15. Completely possessing another Organized religions Holy sites confers 5 MA (Needs to be another FAITH I.E Muslims possessing Constantinople or Christian possessing Bagdhad overlapping Centers of Religion does not count) Also if there exists a an empire with the emperor belonging to a particular faith that faith gains 10 MA. Simply does not make any sense for the Shia empire stretching from Iberia to Constantinople to be stuck at 20