I don't understand why you people are thinking that American cultures couldn't form some great empires? Have you forgot the Incas and the Aztecs? They are a bit out of the timeframe but I doubt Carthaginians would've sent armies across the ocean to conquer some strange lands. First of all, Europeans only managed to conquer the New World because of their plagues and gunpowder. Civilizations in New World and in the Old World were not that different if we consider the technology. So, it would take a 20000-40000 men to start conquering the America. But considering the amount of money and supplies needed to support that money..
Any empire in the Europe at that time would've been bankrupt very fast! They couldn't support larger armies for a long time in Europe why do you think they could support and army thousands of miles from Europe across the ocean? So, thinking that some Celtic people would colonize the Northern America and Carthaginians would take over Southern America is utopia and fantasy!
There would be contacts of course but no conquests or something like that.