The 16th Century saw the emergence of the first truly global economy -- so much more than Europeans crossing the Atlantic, it saw Catholic Europe, West Africa, the Ottoman Middle East, India and the East Indies, Ming China, Japan, and vast swathes of the Western Hemisphere bound together by trade (and empire) in civilization changing ways. And these trade links faced the sea, meaning that so long as a power had a sea port, their access to world markets weren't determined by who their land neighbors were (as such, I've taken to calling it "maritme globalization" to distinguish it from "continental globalization", but that may just be me).
The challenge here -- how can this transformation come about earlier, by at least a century, though preferably much more?
One thing we need to get out of the way -- the answer does
not depend on developing martime technology earlier; if nothing else, OTL Vinland proved that. But by that same token, just saying "Vinland" is absolutely
not an answer; while crossing the Atlantic in some form was demonstrably
possible as the 11th century, it wasn't close to economically feasible in the long term, certainly not in a way our challenge here requires.
CONSOLIDATE: While we're at it, thinking about some common WIs of the late Middle Ages, I'm wondering if they would help or hurt with meeting this challenge:
- there were no Crusader States (in the Middle East)
- the Crusader States were more successful
- the Mongolian conquests didn't happen
- the Mongolian conquests were more successful (took Egypt, etc)
- the Ottoman Empire never rose