I recently started a game in 867 and began reforming every religion i could find and save. That led to...interesting events. Currently Lithuania is a massive Superpower occupying most of the baltic and the only stable nation in the world to boot.
In England Irish Norsemen, Bretonnian Norsemen, The Irish Kingdom of Alba (formerly Scotland) and The Welsh Christians of Gwynedd are duking it out in constant holy wars, host invasions and christian/norse/culture uprisings. basically so far nobody could claim the core of england itself.
Finland flourished with reformed suumonesko and while no longer a major power (they never removed gavelkind, which led to a slow splintering of their massive northern realm) is still a threat to Russia and Lithuania.
Yes Russia somehow managed to stay and form a somewhat useful nation. That is they are constantly breaking up in succession wars only to reconfigure themselves either as ruthenia or rus. At least they hold the tengris at bay.
France is broken into Lollardist France, which is slowly shrinking away, being eaten bit by bit by the HRE,Aquitaine and Norse Brittany. Holy shit Brittany is awesome in this game. You remember that small Jarl of Nantes in 867? The Haesteinning dynasty? Well they not only occupied brittany somehow but also Cornwall, good chunks of england and a good part of Northwestern France. They lost it however but are still strong and kicking.
Reforming the African Faith has proved impossible. While i united all of De Jure Mali, i can't bring down the Idrizid blob which has expanded into spain and eaten the umayyads who were losing against the reconquista. The only remaining question is how long it takes them to expand through the various spanish duke- and kingdoms and into Aquitaine.
Orthodox Faith is nearly extinct. While the ERE is standing strong (at least so far) and has even expanded onto the Krim and a bit into the Syrian/Libanon Area, they have converted to the Miaphysite Faith. The only somewhat Orthodox nation remaining is Bulgaria, which is dealing with constant paulician rebellions and barely holding back the occasional ERE or Cuman Invasion.
A Curious Novum is the Nation of Abkhazia. a broken off part of the ERE they are Iconoclast Georgians which managed to push back Cumania and Abbasids to occupy the Area of Alania and Georgia. I renamed them to Abkhazia again after they formed Georgia just because they were so awesome. Every now and then the Alans break off and stay independent for a few years befor etheir king gets lynched and the Abkhazians take over again.
Central/Eastern Europe is interesting in itself. the Magyars managed to not only occupy hungary but also Greater Moravia and then all those Tribes on the Baltic. Truly massive, they were pushing into the HRE until the Pope called a Holy Crusade and i managed to reform the Slavic Faith.
The Result was the complete breakdown of hungarian culture and the Tengris in the region. Hungaria became Pannonia: a Bohemian Christian State. Greater Moravia was rebirthed as three county dukedom moravia and the Polish Tribes regained independence although none of them has managed so far to take the throne of poland, they are still independent and nobody who tried to mess with them suceeded so far.
The last vestige of Hungarian Tengries has formed the Kingdom of Pomerania and covers the grand total of one county. Lodged between the HRE and the Fylkir of Denmark, it is the battleground of the now constant holy wars and invasions between the two nations.
Denmark is a prosperous Theocracy. Though not so by game rules, its Elective Monarchy and the Fact that the RUler is a Fylkir and always calling Great Holy Wars against HRE makes it one. None have succeded so far , since Sweden is dealing with the War of the Hundred Queens.
You heard that right. For some reason Sweden has only Queens, which are in a never-ending civil war with each other. By my last count they were in a civil war for now well over 100 years.
Norway while a considerable pwoer has lost much influence due to the Byzantine Empire deciding to go "fuck it lets take the Faroers" and consequently also occupied iceland. Just because. Now they are trying again and again against all reason to retake them and suffer from about as much succession wars as Sweden.
And last but not least my favourite ahistoric nation in the game: the Priest-Kings of Orvietto. In the year 900, during the Second Great Scourge of Germany by the Danes, a Norse man named Oddr somehow managed to occupy four provinces in western italy, directly next to the Papacy. To top it off he also snatched a Castle directly in the City of Rome.
That dude was made of such sheer awesomeness that he survived 1 Crusade and several incursions by the HRE. When he died however the Priest Kingdoms fell apart and all remains now is a single county in Pisa, stubbornly clinging to live in this very hostile neighbourhood of the HRE.