This would be very interesting. To name one butterfly, NA would have a much higher native population due to less diseases being spread around.
I've always thought it would be interesting if relatively small groups of Vikings continuously traveled to Vinland and beyond, bringing horses and diseases, while the rest of the world went fairly as IOTL. Then, by the end of the 15th century, when somebody (I suppose butterfly effect would prevent anybody notable from surviving) 'discovers' ~America~ for the rest of Europe, 3 or 4 hundred years of immunity would be built up in the natives (at least in North ~America~).
My earliest TL (which I have never and probably will never posted) had to do with a "secret" group of such vikings, which had absolutely no effect on the rest of the world, and Columbus hits America on schedule, but the natives of the mainland aren't pushovers like on say Hispaniola.
In it, the Vikings and the Skrælings had a very friendly relationship, which is itself probably unlikely IRL. The Vikings had colonies along Newfoundland and Labrador, and had in very small groups traveled and settled as far south as Nova Scotia. All under the rest of Europe's nose.
The most unrealistic part was that both North and South ~America~ were pretty much on par with the Europeans, or various parts of the Middle East.
I wish I still had the text file, because I'd post it and have people help make it realistic. Unfortunately, I made it before I even knew alternatehistory.com was a thing.