Well, rubber, if someone brings some back and someone else accidentally figures out how to vulcanize it.
Then again, not everything the New World has to offer is GOOD FOR YOU. Tobacco might be a draw.

(Or a chaw.

And then, if a Roman trireme makes it all the way up the Amazon to it's head of navigation to Moche without it's crew getting sacrificed, coca.
Coca keeps them coming, that is if the rats don't eat it all before the ship makes it back to port.--it's that habit forming. But bring back llamas. And alpacas. And vicunas, if they survive the trip. And sweet potatoes. And potatoes. And quinoa. And guinea pigs. And guinea fowl. And turkeys. And bring over chickens and wheat and sheep and pigs and cows. Got a big enough ship for that?