What if some tiny bit of geography and geology was different so that the area of Vinland (L'Anse aux Meadows, Newfoundland) settled by the Norse around 1000 AD had had and obvious amount of usable Iron?
I've read that the reason the far western and later Greenlandic colonies failed was the distance they had to travel for trade of iron tools with Norway.
So if a nice handy deposit was found would they have stayed and grown, would more have come. Or at the least would the Greenlanders have moved south, not back east when the little ice age happened?