I recently decided to try a mod that puts (fairly well fleshed out) Celtic paganism, Egyptian paganism and Hellenic paganism into the game, and I'm having quite a bit of fun playing as the king of Pictland this way.
Since the only place in the Charlemagne start date that's naturally Celtic is just two counties large I elected to start as a custom Celtic King of Pictland. No Celtic vassals that way, but I thought I could handle a few Catholic grumblers.
After revoking the sole ducal title in my realm after a civil war and handing it off to one of the other catholics who had previously been vassals of the duchy, I was lucky enough that the chiefs with a claim on the title waged a civil war over it while I was conquering bits of Scotland that weren't under my control, which was a little tricky as my tribal vassals wouldn't help me fight other Catholics.
When the civil war ended all my Catholic vassals had run their armies ragged and I was able to revoke every Catholic title in the land without a single peep of rebellion and replace them all with Celtic chiefs and high chiefs. Cue unstoppable pagan influx into the individual counties of Ireland, and a precision strike to seize a third holy site from the Welsh allowing me to reform the faith. Sadly my initial king perished in the fight against the welsh, leaving his 15 year old son in charge of his lands.
Lacking sufficient moral authority to reform the faith at the time I waged a long series of conquests against the Welsh, Irish and the petty kings of England, slowly raising my authority until at last I was able to reform the faith and install a leader among the Druids.
Ruling from my tribe in Fib I was able to set the Picts onto the Feudal system, cementing my rule over the tribes once and for all. By this point Ireland was conquered, Wales all but assimilated and England a series of Petty Kings fighting among themselves as my armies ground into them in Holy War after Holy War for each separate Duchy as I slowly gathered enough counties to create the Empire of Britain, making me the de jure lord of the British Isles.
Then I died.
This left my 14 year old son in charge. As I hadn't had time to change the Empire's succession laws he was left with my personal Duchy in Pictland, and the Empire title itself, but lost the Duchy of Essex to his brother. Stupid Gavelkind. The Kingdoms of Ireland and Wales had been delegated to an Uncle and a vassal lord by his father already, and fortunately didn't try to break off during the regime change.
Now with Druidic mercenaries and Holy Orders clamouring to reclaim what little of Britain is not under my control I continue to crush the petty heathens of the Cross beneath my armies. In time the men of the Cross will return to try and steal what we have reclaimed, I just hope we're ready for them.