Personally, I don't know why everyone's so excited about Vikings. You can't have Viking raids in a grand strategy scale game except as events, and that's so dull--it would be far better to do it in a small skirmish game like Mount and Blade. You're right that we should pay more attention to the Lithuanians, who are (Tengrists aside) virtually the only pagan group who had any success in this time period.
There is a lot of interest in viking culture, unbeknownst to many, through their conquests, raids and indeed settling, they shaped much of Europe in one way or another. So yes I find their movements fascinating
Norse mod does viking raids without events, enjoyable ones too, as land isn't just being gobbled up all the time, you have vikings acting like vikings. The HI bonus buildings they got, at the cost of garrisons, also gave them a lot of staying power. Raiding for pillage/prestige would be a nice game mechanic for many pagan groups, either for slaves, for food etc. Same with trading amongst groups, tributes, bartering, all that good stuff that would also make some headway into a trading merchant republic dlc too...

, perhaps with a +50-100 years the other way on the end date for that.
As i've said before war should be different when declared on pagans, its not an organised army so much you'd be facing, you're facing the population too, much more uprisings and uncertainty. Holy orders should be available, probably not called holy orders but warbands, hordes whatever you like, whatever fits. Perhaps even a new massed milita or tribal unit type to represent a backlash, giving them a bonus when fighting over previously held land or in a defensive war.
Sometimes perhaps the new religions are welcomed in, but still having the population holding their own beliefs for hundreds of years after you've arrived. War might be much more common between pagan groups as well, raids, border disputes, things like that, but relations moving upward more quickly as well after they are done. - Especially when the HRE decides to just squash tribes one after the other. I don't see why all pagan groups in the area wouldn't want in then, not just a specific sub group. The wars could become a mish mash of all groups after the land, simply exhausting the doomstacks after a while.
Anything can be done, it's just whether it's worth being done or not. Time to Money - Speaking of that, that should be the primary motivator in the AI's decision as well, holy wars are easy to fire off sure, but taking and 'holding' that land should be a struggle.
Turning the clock back a little way would be awesome, as it gives me a completely different scenario to try, it's work sure. An alternative scenario is quicker and easier to do, one where more pagans exist at the game start, so that is also an option people could think about.
And I don't get all theese debates.. First people were screaming "REPUBLICS BEFORE INSIGNIFICANT PAGANS", in this thread it looks like "REST OF PAGAN WORLD VS. VIKINGS!!!!11!11!".. Sereosuly, why can't we all just get along?
Because people like to divide the world and then argue about the divisions.