Sorry for long text and possible errors. I hope it all makes sense.
To be honest, if Sunset invasion wasn't a thing, we'd say Paradox would never consider doing such content.
Personal opinions on Sunset invasion aside, we already have some pretty crazy content for those who want it. Sentient animals including dragons, or super crazy bloodlines like the brazen bull. Hellenism could be included in the list, and people were certain the devs would never do it.
A lot of western historiography set the middle ages to begin in 476.
As to the problems arising regarding Christianity, spanning from an imperial Church to the Great Schism, they could be solved by adding new and dynamic religious features, which would be welcome regardless of the start dates.
Feautures such as councils of course.
And Islam is an excellent candidate for that. Starting with the first successor of the Prophet, there would be a lot of dynamism.
You would have a realm comprising the conquests made under the Prophet, a horde, and plenty of opportunities.
Not to mention a chance to revise current mechanics such as tribalism, of which different kind are needed. Arabian, african, and Northern European did not work all the same ways.
Adding semitic/arabic paganism as well.
Now..... The reason behind the Iqta government, is because the ultimate authority is God. The caliph is his vicar, but not king, because only God is king (i think i got this right).
This means that together with a revamp of technology in general, there could be a social ideology slider (probably a poor name choice).
Instead of having Iqta hardcoded, it would be interesting if a muslim ruler could start shifting the ideology scale and tightening his grip on the realm eventually becoming "feudal". Now this is going to have good and bad consequences, like the Muslim ruler wouldn't have to use piety to prove his right to rule anymore, however zealous muslims will be upset as the ruler has overstepped his given authority.
On the other hand a Christian ruler, donating much land to the Church (and monasteries) and giving the clergy a position of power in government would shift towards a theocratic government like Iqta, so hereditary but with a few differences from feudal, like improved church relation and taxes. There would be consequences again, good or bad, until some years have passed.
The ideology could shift towards an oligarchy, namely a republic. Rulers could perhaps under certain cultures or religions (mazdaki comes to mind) willingly invite the influencial burghers to rule. This would make you a republic (not a merchant one... Yet). Yus republics!
Now the "ideology slider" might all be good, however there should be an indicator to represent the opinion of a county. For example if the ruler has been a tyrant, and an oppressive one at that, the people of the county might ferment a revolt. If successful, why on earth would they remain feudal 100% of the time?
All of this of course would need clear rules, conditions, technological requirements (finally making good use of the "popular customs", "noble customs" and "religious customs"), a good way to prevent players from cheesing it, clear restrictions and consequences if the changes are radical and rushed.
Dynamic cultures anyone?
If you read to here, have a blushy face
