Here are some ideas...
-With a PoD of 1400 I suppose you have time to invent vaccines, especially if you delay the discovery of the Americas.
-Having Cortes and Pizarro fail may discourage future colonial ventures. Then again, having these areas under Spanish rule as opposed to other European powers may help minimize settlement.
-The French are often said to be light colonizers as opposed to the English. A weaker England and a stronger, more colonial minded France may help limit settlement.
-No reformation would mean no New England, removing a major settlement colony. It would also remove the Huguenots. This might limit English colonialism to simply economic ventures like Virginia as opposed to trying to recreate Britain in the Americas.
-Greater trade could mean greater access to guns, allowing the natives the ability to resist colonialism and therefore settlement.
-OTL there were Europeans who would run off and join the natives. You may count them as settlers, but it might allow you to boost 'native' populations and strength while preserving their culture if this occurred more often.
While inoculation has been around since the 10th century in China and/or India, true vaccination against all of the diseases the Europeans brought would be difficult. I can see see it happening ahead of its time in the case of smallpox and it would certainly help save many lives, but measles and those others would be more difficult to fight off. That first wave though will definitely be terrible, especially if it brings a cocktail of diseases. Missionaries might try to vaccinate the natives in return for conversion?
The failure of Cortes and Pizarro would certainly help, Pizarro being the more likely of the two to fail since the Inca were located on difficult to navigate terrain. Cortes failing might only slow the destruction of the Aztec Empire, but it might better preserve the Nahuatl culture and maybe even preserve Tenochtitlan. The Inca might lose their northern lands, but if they can preserve their core areas in what is now Peru and Bolivia I would count that as meeting the challenge's requirements.
Agreed that the French were lighter colonizers, but perhaps a weaker France and England would be more beneficial. Maybe if they expend most of their funds exhausting each other with war or intercontinental trade competition that North America does not cross their mind? Or at least, trade companies would prefer to settle the southern areas around the Caribbean as opposed to the North (Jamaica, Barbados, and Haiti Redux, except expanded)
Fascinating PoD with the No Reformation. It definitely would help in limiting the amount of settlers willing to go to New England. Would not have thought of that. But how do you prevent it, or perhaps contain it? It was very revolutionary and spread quite quickly.
Greater trade was what allowed the Iroquois to secure some power, allowing them to defeat their weaker neighbors through guns by trading for fur. But what comes then if the natives don't have a good to trade to the Europeans? Then they lack the economic power to request for guns. It would also require those guns to not be used against the Europeans for some time.
I would definitely count Europeans who joined the natives as natives if only because most tribes would expect them to take up their customs. It definitely boosts their population in terms of numbers; if they are lucky, they might even net a skilled artisan, a trader, or perhaps farmers who know how to grow Old World crops.
Here's one I've suggested before:
While I don't think Mansa Abubakari actually made it to the Americas, lets say as a counterfactual one of his ships makes it to the Americas. Are any of the sailors alive at this point? Doesn't matter. What matters is a stoway has survived, an Aedes aegypti mosquito carrying yellow fever virus. The mosquito flies out of the ship and lucks out by finding some hosts, a troop of monkeys on the beach. It bites them and infects them. Now introduced into its sylvatic cycle, where monkeys are infected by mosquitoes living in forest canopies, the disease spreads across South America into Central America. In some areas humans are infected incidentally by the disease cycle; in other places such as Mesoamerica, human populations are so dense that the virus establishes a human to human cycle of transmission. Either way, the sudden introduction of a large population that is naive to yellow fever-for example, an invading Spanish army-will result in an epidemic in the latitudes between Rio de Janeiro and Veracruz. Mass die-offs meet the would be invaders by the time the 16th century rolls in, and large parts of the European conquest are averted with no Black Death 2.0 involved.
This would definitely help stave off colonization if most of the people died to yellow fever. Though, if yellow fever gets carried across, why not malaria too? Or other mosquito-transmitted diseases for that matter.
Maybe just enough Norse colonization of North America’s Atlantic coast in the Middle Ages to introduce Old World diseases to the rest of the continent and recover demographically from the resultant plagues before other European powers show up?
Hmm, I feel like this has been discussed before, but it is doable to an extent. Trade was less developed in the Northeast than say, Mesoamerica, and even more so since the Vinland colony was located on Newfoundland, isolated from the rest of the continent. It would require a southern push. Maybe if Greenland is abandoned and they all fled to Vinland, and from there Nova Scotia, or the St. Lawrence Valley, maybe Massachusetts Bay? That would exponentially help spread the early wave of disease, though they might mutate differently from Old World versions of the disease.