Okay, before everyone flames me because the Vikings weren't Christian, hear me out. I am suggesting that perhaps the crusade/voting for pope thing could be altered (especially considering that crusades don't appear for about 300 years) to be a call to sack a certain settlement. The votes could be for a viking high king (which they had).
There were several cases historically of Viking high kings calling on their subservient neighbors to assist them in attacking a city. Some of them even attempted to sack Miklagard (Constantinople). I know that you guys are probably steeped in work already for this mod (it looks great, by the way!), but I just thought that this might add a new level of gameplay. Especially if you could return the "sack settlement" command to its previous and historically accurate use in RTW:BI (destroying almost everything, taking the cash and leaving the settlement in rebel hands).
I know nothing about scripting, really, so this may be impossible, but the Imperial mod seems to have altered it to elect the Holy Roman Emperor. Use it, or don't. I leave it in your most capable hands.