One problem the Scandinavian countries have had is a lack of people compared to the Southern European countries. When they tried to fight Russia, Prussia, Poland etc, this lack of manpower always held them back despite having some very good leaders.
Now what if the Vikings who traveled to Newfoundland had gone farther south? At that time some types of corn were being grown in the North East parts of America.
Could the Vikings have successfully introduced this new vegetable to Greenland and Iceland, and finally to the Scandinavian countries? If they could this new food source could cause a major population boom, much like the potato did in Ireland. Would this reduce the Scandinavian under population long enough for them to really expand south and east?