Has anyone got the When the World Stopped Making Sense mod? It's glorious. Bumps your start date back to 476 at the earliest (meaning a lot more religions to play as), and provides mechanics to simulate the age of migration and increases the amount of provinces in the game significantly to make Northern Africa and Arabia more interesting. My only complaint is that it crashes every so often, so you really can't play on ironman, and that the unfinished Rise of Islam events are disabled by default as they're still working on them.
Some interesting states from an Observer Game I've been running;
-The Great Seas Merchant Company: I'm not sure how this came into existence. Maybe it's a default state at the start. But anyway, it's a titular title that handles succession the same way as a mercenary company does, in charge of the merchant republic of Scotora. They are Messalians, and have managed (through conquests and being a place where claimants can convert to Messalianism) to not only expand significantly but to spread the Messsalian faith to all of Yemen and the Axum Empire. The Preeminent power in the Messalian world, and soon will become the main branch of Nestorianism as the remaining Nestorians are all living in provinces ruled by other faiths.
-The Bolghar Khaganate: This is an example of game mechanics gone horribly right or horrible wrong. Essentially, the Volga Bulgarians invaded what in the base game is Khiva and Turkestan, forming a massive empire, then somehow started to acquire event troops out the arse. Seriously, at one point they had 250,000 event troops; thankfully they almost all died of attrition before they could utterly break the game, and this bizarre process of troop generation seemed to be unique to a single Khagan. Anyway, they eventually managed to wrest Persia itself from the Sassanids (one of the two strongest powers outside of India at the start) and fight off the massive Byzantine holy war that erupted as soon as their borders touched. They are Tabitans, which is the pagan faith practiced by the Scythians and Iran pre-Zoroastrianism. They managed to reform the faith, so expect a lot more Bolgharian shennanigans in the future.
-Kingdom of Eire: Essentially, Irish Celtic Pagans made a comeback and pushed the Christians out of Ireland. They then conquered Scotland (which went independent due to Gavelkind) and Norway (ditto). Now they're halfway through the conquests of Wales and England. Celtic paganism is strongk!
-Visigothic Empire: The Visigoths were already huge. Then they decided to eat Burgundy, and got so huge they formed an Empire title. Fortunately for the Nicene Christian world, they converted back from the Arian heresy to mainstream Nicene Christianity; otherwise, the Arians would have outnumbered the Nicenes within a few generations as conversions hit and the Visgoths made the Holy Wars that destroyed the other Arian states unwinnable. The Visigothic Empire collapsed around the turn of the 7th century, but left in its wake numerous Nicene Christian kingdoms on every front.
-East Roman Empire: The undisputed strongest state in the World (well, disputed only with the Bolghars, who may well be becoming stronger due to bugged event troop generation) with over 60k men, the ERE is the main Nicene Christian state and controller of 4 of the 5 Patriarchs. They have continually expanded, whether in pushing the Sassanids out of Armenia (which remains an independant Zoroastrian state), expanding into Nubia or their successful conquest of Astoristan (think Southern Mesopotamia) which finally drove their ancient Zoroastrian rivals to near irrelevance. The ERE became an elective monarchy due to faction demands, and its Emperors seem to fall down the stairs and land on multiple knives frequently. These two things could not possibly be related.
-The Sassanid Empire: The main Zoroastrian power and once an equal to the ERE. However, constant wars with Rome and the rise of the Bolghars rendered it almost irrelevant. Currently, it controls only the de-jure Kingdom of Arabia (in the mod, there are 4 kingdoms where Arabia was; Hejaz, Oman, Yemen and Arabia. This is due to there being a lot more provinces there) and following the fall of Cstephion to the Romans has moved their capital to Ridyah. The most interesting thing about them now is that almost all of the land is controlled by a single Queen-level vassal, who looks like she'll be succeeded by another female ruler; in other words, one revolt and the Empressive achievement could be pulled off by the AI, so long as her daughter has no sons of her own.
-West Roman Empire: Surprisingly enough, the Western Empire managed to beat off Odoacer and remains whole. They lost Pannonia, but managed to expand into the Alps, so overall little change in borders over these 150 odd years I've observed. However, their missionaries have spread the Nicene faith to the various Germanic Kingdoms, and only 3 king-level rulers remain who embrace Germanic paganism now, so they've managed to almost eliminate a religion without directly holy warring them.