Viking Invasion, no question.Not only because it fits perfectly as an expansion from the historical time frame point of view.
My main argument would be the size of troops. In Englands 6th to 9th century 200 men were an average sized army every warlord was holding. So it again fits perfectly to the possibilities of the game, which is approximatly providing these troop sizes.
In times of Carl the Great (or the byzantines, whatever), important battles where mostly fought by armies of several thousand men. Realism is a big thing in this mod as far as i understand it. So please dont let Carl the Great or William the Conqueror fight with 100 men vs 100 men.
Idibil & team, just a suggestion: i was thinking of the possibility that you keep the mod as it is (same units, same factions, same map) - beside there is an invasion of dena vikings and other norsemen (maybe new units)!

Like a few hundred norsemen under different kings/warlords (additional factions) are invading the map from time to time, trying to loot the country or get land! man, how awesome would that be!!!

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we can proceed with the history of the character we are playing at the moment, but then a new threat arrives in this land with dragonboats, trying to burn down the island and you have to ally with old enemies to survive the viking invasion. boah, i have to stop imagening before i wet my pants...
gentlemen, thanx for making many of my evenings these days! Respect to your commitment.