The challenge is this: come up with a scenario in which, before the arrival of Europeans, the civilizations of the Americas are exchanging goods and ideas.
Must include: A civilization of Southern America, at least one from Meso-America, and at least two 'culture groupings' from North America.
Is it even possible? Or am I just being ridiculous?
I think it is very possible. And weu only need a single POD, although how to achieve that POD...
We need better ship tech for someone. The Caribbian and Mexico Gulf is pretty ideal for a sailing and raiding/trading civilization. And a raiding people could marture into traders.
Ships trading from the South American Taironas, all the way up the Mississippi would exchange goods and ideas at a furious pace.
But we need a leap in ship tech for that. (Personally, I think their would have been more imprtant to the Americas than horses.)
How to get it? We can get it from the vikings, and that comes with a few additional bonuses. But we only get a few centuries to play with it, and that may not be enough for things to properly settle. We may get something like the sea peoples bringing civilizations crashing down, without time for things to get back into swing again, or even a situation where sailing doesn't really get out of the lag phase.
We can get it from other old world civilizations. Romans, Cartagians, or my favorites, Tartessians and Minoans.
But that may very well come with other cultural seeds, and at such an early point in the Mesoamericans history that their cultures become unrecognizable.
It would be best if it was a native development, but I am at a loss as to how to get that. Because we need a rather large improvement.