I've heard a lot of speculation lately about what the next 4 dlc packages are going to be, and one of the major requests/suggestions is a dlc that would make pagans playable and expand on their culture and society. But it seems to me that pagans would not be a very interesting expansion to the game and would not warrant a dlc expansion for a number of reasons:
Firstly, this time period is one in which pagan culture was in decline and Christianity was sweeping across Europe. The Viking Age ends right at the beginning of the game and no other major pagan culture ever emerged. The inevitable Pagan empires that would emerge in a Pagan DLC would be dramatically unhistorical. To make it more realistic would be to make the pagan factions almost impossibly difficult.
Secondly, pagan culture, no matter how you look at it, was was less institutional and complex than Christian Europe. Social structures were based around old tribal systems with chiefs who often ruled by might not right. The dynasty focused play of CKII would not translate to pagans very well.
I really want CKII to be fully playable and I believe that the pagans are an important part of that. But I would hate to see an entire DLC dedicated to them. A DLC focused around the Northern Crusades that made the Holy Orders (especially the Teutonic Knights) playable as well would be worth my money, but there is no way that pagans alone are worth $9.99.
These are certainly not all of the reasons just the ones I could come up with off the top of my head.
Less institutional means more complex to an extent, because they still had forms of organized government. Iceland had a weird elective government with hereditary titles, Ireland and Scotland had decentralized elective monarchies that ruled over other monarchies.
Your also forgetting the Lithuanians, they lasted for awhile before going Christian and then they inherited Poland forming a Commonwealth that was one of the major players in European politics, until her neighbours wrote some laws saying she ceased to exist in the 1790s.
I read someone suggest that a Pagan DLC should be about the slow conversion to Christianity, and while that wasn't terribly popular on that thread, that would make historical sense, the reason for the conversion was power, Mieztko went from being a pagan farmer or something to a Catholic King, the Litva got people to stop invading them and legitimacy, plus new institutions largely under their control. I think there should be a choice, a hard fight to stay Pagan or a slow conversion, but it needs to be more balanced, the HRE was not centralized enough to be the steam roller it is in game, converting all the Wendels and taking the Baltic Coast was not a quick thing. Hell, the whole coast tends to go by 1100, while in history the invasions went into the 1400s, with the power of the Teutonic Order and the Scandinavian Kingdoms behind them.