The biggest potential butterfly I can see here won't affect the Romans or Indians directly for centuries. AS people say, the Romans are going to wind up assimilated. This difference will come back and bite Cortez in the ass though. That is to say, disease.
Much of the conquest of the new world was greatly eased by European diseases going through native populations like a chainsaw through wet cardboard. If the Romans bring European diseases to the Americas early, the people most susceptible to those diseases are going to die a solid thousand years too soon, and the genetic predisposition to be resistant to those diseases is going to be present in all the survivors, who will have time to be fruitful and multiply and get the population back up to par before Explorers show up.
This won't mean immunity, however even slightly better odds will tip the scales dramatically. In many communities estimates are that 95% died to various diseases. Imagine if that was reduced to 75%, still staggering losses but it means that small Indian Village is now five times bigger and the raiding parties the westerners faced in the OTL have now become small armies. If the death rate was, say, 50%, still very high, they might have ten times their number and it's more like mid-sized armies. This would seriously curb any hopes of western expansion and make them far more difficult, perhaps turning the situation into one more like Africa where the natives were never truly displaced and remain the majority to this day.
