Here is Dev Diary 98 of Crusader Kings III.
It's mostly talking about the content that comes for free in the patch coming alongside Fate of Iberia.
Clan rulers get a few new mechanics in their contracts. Three of those are based on Muslim laws: Ghazi (vassals are encouraged to go on Holy Wars but give less levies), Iqta (less tax and levies but better Men-at-arms synergy) and Jizya (tax non-believers, leaving you with more money but less levies). Jizya is linked to the tenet of the same name, which is called Religious Tax in other religions. So it might be possible for non-Muslim clan rulers to get it to.
More interesting for modders is that there is now an option to arrange a marriage promise as part of the vassal contract when clan. I expect modders to try to make it work outside of the Clan government and in Fantasy Total Conversions. Feels like something you should be able to set in Game of Thrones
Another big addition, and one that I particularly like, is the fact there is now a new Faction in the game: Dissolution. Simply put, vassals can now push a rebellion to destroy their liege's primary title and all equivalent level. In the context of Fate of Iberia
this was added to better represent the collapse of Al-Andalus, but it basically means now that big Empires have a risk of collapsing on their own. Which is huge. Dissolution factions are supposed to become more likely in situation that would foster crisis: short reigns, low cultural acceptance, war losses and unallied vassals. Popular heirs reduced the threat of having such a faction, as does having an elective succession. Vassals are also more likely to go for claimant faction if they have a claim than for the Dissolution. I must say this is an addition I particularly like because it adds a challenge to those who want to build huge Empires: you'll know have to watch out and control your expansion if you don't want to collapse like the Roman Empire. And I can't wait to see how modders will work on that faction...
Finally, among the big shots, the Dev Diary touched to new Shared Head of Faith mechanic, which I'm happy to learn is free. Sad thing though, this will only be possible via the taking of a special Rite tenet... Feels like a mistep for me. Sure it's more a feature for roleplay (gaming-wise it's more advantageous to create your own) but having to sacrifice one of your tenet slot to keep the same Head of Faith isn't ideal. Oh well, modders are probably going to find a work-around for that.
There are interesting things that come from having shared Head of Faiths though. Creating a new branch of Christiannity that has the Ecumenism doctrine for example means you'll keep the latter if you pick the Rite tenet. Islam for its part get to choose who they think is best suited for the Calpihate, whether by picking one of the existing ones or by simply claiming it for their own benefit. And I'm pretty sure modders can find others.
The rest of the changes mentionned are fairly minor by comparison (they're mostly historical adjustments). And the final part of the Dev Diary reveals the new Achievements that are coming with the release of Fates of Iberia