This already happened, Greenland are part of the North American continent, and it had a Indo European population for 500 years before Columbus.
samcster94: "For obvious reasons, Old Norse is excluded as it is obviously Indo-European and was spoken in the Americas before 1492 and earlier European contact is excluded. South Asian colonization is allowed though, it just cannot be a European power."
OP asked for a Indo-European migration exept for old Norse and europeans in general.
This does restrict the amount of possible scenarios. If Indo-European expansion into the Americas cannot happen via Europe, then it will have to come from somehere else. Some possibilities i mangaged to come up with.
- Maybe Indo-Europeans can establish themselfes in North Africa? From there they may then colonise the Americas. North Africa has had Greek, Latin and Vandal settlements. Maybe if one of these manages to establish itself in a stronger and more lasting position,then they would later be able to spread to the Americas
- Indo-Europeans migrate into Subsaharan Africa and from there spread into the Americas. The Indo-Europeans could arrive there maybe via the Sahel, a flat steppe where the Indo-Europeans know for their horses could cross Africa.
- An eastward Indo-European migration in Eurasia. As these Indo Europeans reach the Pacific, then they may cross the the ocean and land in the Americas.
- Indo-Aryan eastward expansion. Similar as the previous scenario.