Suggestion to extend the slavery mechanic (or outright trying to inspire you to make a slavery mod lol), making it primarily tied to culture instead of religion, and some "special features" for a couple of religions, it's my wishful thinking of course, i'm just throwing it out there since it dawned on me, to see if it stimulates your own ideas for your modding, which i'm falling in love with

Norse (Only Norse, not the whole north germanic group)

West, Central and East African (all 3 groups, Entirely)

Arabic (whole group) - Gets more cash from slaves with African cultures, foreign prisioners can be made into courtiers to be employed (you can't press their claims if they are not of your religion, can't grant them land if they are a primary heir, and if they don't like you enough they will attempt to escape)

Altaic (whole group)

Iranian (whole group)

Hungarian

Nahua

Han - Can Only Sell Non-Han as slaves, Unless convicted with murder

Israelites can't sell other israelites

Both Indo-Aryan and Dravidian groups

Alan and Assyrian

Norman

Anglo-Saxon and Saxon

Irish

Vlach

Some Goverment Tweaks:

Nomads can always sell slaves, even if religion disallows it, but so it gives hefty prestige/piety penalties and opinion malus towards felows of the same faith

Tribals can always sell slaves unless religion explicitly disallows it (see below)

Iqta gets more cash for military/stwewardship educated slaves, reflecting how they were heavily used for war and bureacracy

And instead of making religion the primary requirement for slavery, make it tweak/override the cultural avaiability

Muslim can always sell, exept for Sayyid and Mirza charachters (unless branded as decadent)
they can engage in "large scale" slavery, which operates as a stacking character trait that's gained when looting provinces or crushing uprisings, it can be spent to trigger county modifiers or to raise retinues, but you run the risk of slave uprisings (plus slave armies have really low morale)

Christians can't sell other christians into slavery (not even when heretic, Norse, Normans, Anglo-Saxons, Saxons, Irish and vlachs are exempt)

Jews can't sell other jews (not even when heretic)

Hindus can't sell the 2 upper castes (Vaishyas are fair game though) and as any other religion, selling the 2 upper castes will outrage hindu rulers

Confucians get more money from selling eunuchs(for use in bureacracy) and underage females (for use in domestic servitude, as it was a sign of status and prestige in tang china to be employ mostly young females as servants)

Jains and Bons can't sell slaves at all (not even when tribal)

Buddhists can only sell them if they are nomad

Manicheans can't sell other manicheans

Mazdaks can't sell slaves unless nomad

Zoroastrians lose piety when selling slaves

Following pagan faiths can always sell regardless of reforms, any other pagan can sell if religious tax/seabound is enabled,

Aztec (And any religion with bloodthirsty gods) can always sell, but fellow aztecs will disaprove (you could be feeding the gods!)

Germanic can always sell, in adition chance to generate a random prisioner every "Tick" when raiding (prisioner is lowborn and of culture/religion of province) selling clergy of other faiths gives prestige (nuns/monks and theocrats or their courtiers, not sure how to synergize this with iqta and monastic feudal though)

Tengri can always sell, when a nomad raizes a holding prisioners can be generated in a similar manner to above

African can always sell slaves, when you win a duel against a relative for a title you can decide to sell them instead of killing them

Zunist can't sell fellow zunists nor people of your dynasty (not even distant relatives or bastards)

Hellenic can always sell slaves, they can engage in large scale slavery like muslims (but can't raise slave armies) they can use male capable prisioners to perform gladiator games which gives prestige, little hellenic opinion and reduces unrest in one owned county and give a slight tax boost if it's hellenic, the better combat the prisioner has, the more prestige their owner gains, if a character gets "insuficient funds" they can get imprisioned by their liege, hellenic rulers don't release their war prisioners uppon victory, instead they can use them to perform a triumph, the more prisioners taken in the war, and the higher their rank, the more prestige the triumph will give, hellenic rulers can sell foreign prisioners of ANY rank this will outrage everyone in their realm, as well as minor outrage from anyone of their religion and their culture, and cause lots of unrest in whatever land you might have taken from them in the short term

more details about large scale slavery- it can be stacked up by sieging down holdings, crushing uprisings, winning wars, depending on the size of the realm defeated (in this case it gives a temporary economy malus to the defeated party, or, if the defeated becomes unlanded, gives a huge bonus to the victor)