Essentially, how can the Americas be contacted in a way that allows its inhabitants to maintain their independence for as long as possible? What circumstances do you think are essential for the natives to come out of any ATL Columbian Exchange in a more prosperous position than OTL?
I personally think that naval technology should be capable enough to cross the ocean but also rudimentary enough to be replicated. Contact should also be sporadic enough to allow some recovery from diseases and the social turmoil they entail.
Polynesian navigators, or the capture of a Spanish expedition Gonzalo Guerrero style? Vikings, Basque or Japanese fishermen, Swahili traders, or Berber pirates? Any PODs of your liking as long as the native peoples remain independent for much longer than OTL.