1. Imported weapons and armor.
Weapons and armor from other European cultures not included as factions could become available at the traders, but not available as loot (not assigned to any troop type). They should be very expensive and very rare, perhaps even only sold by the Book Merchant or through quests. This would assure that only the player character or his/her companions would be able to equip them so that the game would not lose its feel (although use of foreign armor by Vikings is mentioned in the sagas, and in those times, well, armor was armor, not a uniform).
1a. More weapons.
The atgeir or hoeggspjot (also called the Viking halberd) for a more native hafted blade feel, or two-handed wooden clubs.
2. Northern Frankia / Normandy
Seeing as there already is a part of the map with a section of the northern coast of modern day France, Northern Frankia could be introduced as another faction to the game. Historically, these lands were raided by Vikings since the second half of the 9th century and Normandy was specifically a fief held by Norsemen (Rollo, among others). The inclusion of this faction could spice things up for the Frisians, as well as the south of England. Their troops could distinguish themselves with kite shields.
3. Long shot - the Baltic Sea
Realistically speaking, I don't think this is ever going to make it into the game, but here it goes: the Baltic Sea, with Sweden, Gotaland, Pommerania, Old Prussia, Rus and Finland. All of these lands were frequented and raided by the Vikings. This could be coupled with the inclusion of tropps like the Jomsvikings, Estonian Pirates or the Varangian Guard veterans as mercenaries.