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I just thought of this now, why didn't they bring smallpox or another disease they had to the new world? They had settlements and certainly had the diseases that could have spread through the new world.
My proposed effects are (obvious):
Millions of Natives Die and the New World is Ravaged.
More permanent Norse colonies (lack of hostile natives).
When Later Europeans arrive they are recovering (500 year interval) and have an immunity, they may be able to survive longer against invasion (especially against conquistadors).