So the Indo Europeans originated in Eastern Europe some time in chacolithic ((copper age)) to early bronze age. They were some of the first to use chariots or even horses in general outside of simply eating them. It is thought due to an unstable climate that they migrated into Europe and India. Their languages became the root language for almost every language in Europe and North India and their Gods became the inspiration both Hinduism and most all the European Pagan religions. However their influence does not end their, they also popularised the idea of Individual as opposed to mass graves, the use of chariots, and most patriarchal society root from the Indo Europeans. without the Indo Europeans there are no Latins, Greks, Celts, Germans, Slavs, etc. The only well know Native European cultures today are the Basque who are the only ones still around, the ancient Etruscans, The Aquitani, and the Minoans.
This would be a major challenge but I think thats what really makes it interesting and I'm sure with time and research Mr. Z could make something inteesting at the least. So my suggestion is that the Indo Europeans never migrate or that if they do migrate they migrate East into the Asian Steppes. Why doesnt really matter as we still arent sure but what happens is a drastic change to world history, but if want a definite then just say the climate is more stable in the region as to not facilitate a migration or something like that. Something a bit odd and i'm sure unique as far as fan scenarios go.