Many have actually theorised that I2a-CTS10228 expanded into Eastern Europe with Celtic speakers based on the fact that CTS10228* has been found in France and Germany. It was then picked up by the Proto-Slavs. However, it is certain that CTS10228>Y3120 in the Balkans only arrived during the Early Medieval. This is clearly shown through TMRCA and the fact that no CTS10228+ aDNA has been found in the region. As for where it expanded from, evidence does suggest Eastern Europe due to diversity. A study from this year suggested Southeastern Poland as the area of expansion https://link.springer.com/article/10...20-019-00996-0.