I've been spending an unjustifiable amount of time thinking about the situation in the Americas. (because I live there)(and am given to extreme flights of fancy.)
It was kinda a combination of factors that screwed them over in reality, like a mass extinction of nearly every domesticatible animal on those continents and a near complete lack of idea, tool, and crop diffusion both among the major local civilisations and between them and the rest of the world, and a late start both on agriculture, and on when they managed to breed certain critical crops into easy growability.
So this would be easy to change, have the domesticable animals survive, set up the central Mesoamerican civilisations as enthusiasic seaborne raiders/traders/colonisers to facilitate idea diffusion, juggle the positions of the Pacific Islands so the Polynesian waves of expansion don't peter out before reaching the Americas, and/or just do what is implied right now(what with the existence of the mediterranean black sea) and make Greenland warmer and better able to support a European style population, agriculture and trees so the Not!Vikings, or their equivalent can support early colonization efforts towards North America. (which can either succeed or at least cause idea diffusion)
Honesty even only one of these things would leave existing cultures of the area in a better position to be politically relevant.
Also clearly Terror Birds must survive to be domesticated as hunting, then riding beasts in South America, whimsy demands it.(and maybe some of South America's other bizarre fauna with them)
I'm imagining it now, the Incan empire's or Amazon river civilisation's quick responding terror bird cavalry lined up to charge against the hordes of viking raiders and drive them back into the sea.
