The Vikings are lacking a strong late-game identity and don’t have a powerful unit on most maps.
Additionally, the Berserkergang ability is too expensive for its minimal effect on a single unit.
One suggestion is to incorporate the Berserkergang ability into the base stats for the Berserk unit.
However, a more interesting idea would be to give generic units, like cavaliers, champions, and arbalests, special abilities for the Viking army.
While the current 40hp/min bonus would be too strong for generic units, a weaker version of 10-20hp could be given to all Viking units, making double gold compositions more viable without being overpowered.
While there may be some flaws in the idea, it is worth considering.