Why make the UU a berserker, the vikings already have that in warlords, it would be a shame! An idea for a UU with inspiration taken from Medieval Total War could be the "Obudshaer" problably an english version of the danish "Opbudshær" meaning "Drafted Army", a levy unit armed with halberd, or swordstaffs (a weapon unique for Scandinavia) this is just ideas for gameplay variations. otherwise go for a unit like longboats, the superior naval vessels used by viking era Danes, which enabled them to traverse great sea distances, including the passage to America.
If you would like a leaderhead (I know you've already got three) you could go for the Prime minister of Denmark during the early WWII, Thorvald Stauning, widely considered one of Denmark's greatest prime ministers, Traits could be Cultural, protective? Try Wiki'ing him. But this is just a propsition
, I like your current Leaderheads very much!
Ideas for UB:
Runestones instead of monuments
Longhouse instead of barracks
Danevirke instead of City walls
...