There was also very limited European colonization of Africa and none in Asia or the Pacific.
I suppose what you describe could happen if there were a powerful enemy encroaching on their territory that were to draw their attention away from the Americas and focus it on mere survival. If the common foe also had no interest in the Americas, then contact may take place but not necessarily conquest, as you describe. Of course, I was that foe in that campaign, but in reality, perhaps the Ottomans could serve in that role. Alternately, a natural disaster which wrecks Europe might suffice, although engineering a realistic one which doesn't wreck everywhere else is nigh impossible.
This is related to the "holy grail" I sought when I first got into AH -- I wanted to write a story wherein Europe was devastated by a deadlier Black Death, sparing the Native American civs from wholesale conquest and leading to world dominated by Eastern and Middle Eastern civilizations. When I found I couldn't have a deadlier plague wreck Europe without wrecking the rest of the Old World, I looked for other ways to accomplish the same thing. My next plan was a meteor strike in Greenland during the time of the Black Death which temporarily disrupted the North Atlantic Current, creating a "Black Winter" that severely exacerbated the pandemic. This was admittedly far-fetched, so my next plan was a political one. By this point I was even more out of my league, but with help from Falastur (a member here), we worked out a way in which the death of the Pope and some cardinals due to the Plague could lead to a series of devastating "Papal Wars" across Europe. This in turn could leave the Christian powers more susceptible to invasion by the Ottomans, who could overrun Europe and make it a backwater of civilization. In all plans, the idea was to later expose the Native American civs to Old World diseases and technology via low-level contact with Arab merchants.
Unfortunately, the nuts and bolts of the final plan (i.e., historical European politics) were and are well beyond the scope of my knowledge. Coupled with the fact that I discovered several published AH works which had already dealt with the same subject (using handwavium
), this resulted in my giving up the project.