Ok, an idea for the Vikings / Norse civilization:
Special units:
Longboat - Replaces trirreme. It has one extra movement (exploring advantages!), triples the amount of gold generated by pillaging workboats (a small touch of flavour) and most important of all, adjacent land units / cities gain +2 strenght (that is, it acts as a support unit for both amphibious assaults and city sieges!).
Berserker - Replaces swordman. Pillaging heals them compeltely even on enemy territory (the more there is to pillage, the longer they can substain their invasion). When embarked it can attack coasts as if it were unembarked (no amphibious assault penalty), they defend when embarked (a la Shongai), thus making your fat, rich overseas neighbours unsafe, mwahahaha.
Special ability:
Hanseatic leage
Maritime city states influence degrades at 1/3 of the speed.
City states rewards you with naval units as well as their usual bonus.
Friendly city states generates trade routes with your cities as if they were a part of your own empire.
Because Norse civilizations weren't all about raping and pillaging
and because that would encourage to play as them: use brute force early in the game to secure your sphere of influence, fund remote colonies and find allies earlier than anyone, then work hard to create a economic union able to fuel your economy and leverage your early advantage.