Serbian I1 does tend to be Germanic in origin, although there were indeed multiple Germanic peoples who passed through, including Saxons and Ostrogoths. Do you have anything more specific than "I1," like STRs or additional SNP testing? That might help narrow the source down more, or at least give a more informed guess. West Germanic peoples tend to have more I1-Z58 and East Germanic peoples are usually thought to have had more I1-Z63.