Flubber
Banned
How about religious fervor? That's always a good one. Perhaps the Wars of Religion are much more widespread, while deepening a reliance on faith and a rejection of secular sciences and non-orthodox thought.
But those people employing secular sciences and non-orthodox thought are getting rich and, as Cicero pointed out, wealth translates directly into power.
If new lands are discovered, it's the will of God.
Those new lands were discovered in the 15th Century because of the explosion in secular science and non-orthodox thought.
Alternately, would an epidemic be too ASB?
A reverse Columbian Exchange or one that goes both ways? That will depend on what diseases the New World can produce and we all know that will depend heavily on how many domesticates the New World contains.
Then again, if a plague or plagues ravage Europe to the extent you're considering, why is there even an European style kingdom founded in the New World?