Actually I agree, but only with the fact that achieving the first culture expansion is way too easy
I was thinking of increasing the amount to 30 culture, and moving the "build" culture option later on the tech tree
This does not solve the real problem. It does not change the position of culture in the game, only give Norse a penalty. The speard of region must be considered. Other civils have missionaries when spawn and the cities can get region easily. So the penalty of this change for other civils is much less than for civils without region and can not get region easily. Logically, if you adopt this change, you have more reason to decrease the cost of pagan shrine or give some additional bonus for it.
I understand that you want to let regions have some advantage than without regions. But the advantage is too much: 25% bonus for building from civ, 15% research bonus for monstery and big chance of speard.