This is very difficult to do. Longer lasting Vinland bringing disease and European tech would be interesting. However, we have a 500 AD POD limit, so I'm going to use that instead. We could do something like The Legacy of St. Brendan. Have an Irish monk (they sailed a lot and landed on Iceland before the norse) discover Newfoundland. Later, small Irish colonies pop up. However, they are eventually forgotten by Europe. These settlements will have a massive impact, however.
First, they spread disease, most notably smallpox, which rages through NA. The population collapses, but without European marginalization and expulsion, it doesn't collapse as hard, and is able to slowly recover.
Second, they spread technology. Lots of it. Most importantly they spread:
-Metal-working
-Wheat (possibly onions or something too, but wheat/other grains is the most important)
-Writing (Already in mesoamerica, but they could spread it to the rest of NA too.)
-Domesticated animals, such as sheep, horse, pig, cow and chicken. (Possibly only some are shipped to the colony)
The remaining Irish colonists slowly mix and creolize with the native Beothuk. Meanwhile, their technology begins to spread. Through trade, iron, wheat and domesticated animals start to spread throughout the somewhat-depopulated Americas. The crops and domesticates are massively useful, and help the remaining Native Americans to stage a demographic comeback. Metal is first only traded, but eventually the knowledge of smelting spreads as well.
One question is what happens to Christianity. The Irish would be overwhelmingly pagan by the early 500s, though slowly converting. Whether any Christians at all arrive is up for debate, and it's possible the few that do arrive have their beliefs syncretized by the new Irish-Beothuk culture. It's also possible that Christianity could slowly spread through the Americas, which would be interesting to see if the Europeans later re-discover the continent.
The population of the Americas slowly booms with all this new technology, and civilizations grow larger and more powerful than ever before. Smallpox still rages among them, as well as a new disease or two they get from their domesticates. When the Europeans discover the Americas, they find large, iron-age empires, farming wheat and raising livestock. With resistance to smallpox built over the course of 1,000 years, the Native American population is much larger, and remains as a plurality.