The Viking era. Imagine a map set in Norway, or Denmark, and being able to build a longship, recruit a crew, and pound across the North Sea to raid Northumbrian villages and monasteries. Norway/Denmark and coastal Northumbria would be two different maps, and Northumbria wouldn't have much depth inland to allow the coast to be as long as possible. Crossing the North Sea wouldn't be easy either, it would take time and risk bad weather and simply getting lost. And perhaps back home in Scandinavia, quarreling jarls result in battle and bloodshed at home.
More of an entirely new game than a mod, though. It would require lots of new assets, all new armor, clothing, and weapons, new combat tactics cough SHIELD WALL!!! cough, not to mention a boat/ship system. But I can dream, can't I? It would certainly be more refreshing and interesting then a medieval reskin like the Hundred Year's War or the Crusades, and the naval aspect would be immense.