The best case scenario would be a a short-term unification of several Germanic tribal groups, who could have invaded Gaul, and established a permanent nation or grouping of tribal kingdoms in that region. Merging with the more culturally sophisticated Gauls could help their cause. It won't be a purely Germanic culture, however. And either having close links with, or taking control of the Greek city of Massalia (modern Marseille) may be useful.
The Germanics had been steadily migrating beyond the Baltic as early as 800 BCE. The Bastarnae was one such group. Their territory was located north of the Carpathian Mountains, so they would have enjoyed relatively close trading links with the Scythians and the Greeks that lived around the Black Sea. The Baltic shoreline was, and still is an enormous source of Amber, which often found it's way to the Greeks, Phoenicians, and the Egyptians. So they weren't quite isolated from the urban cultures of the Mediterranean.
While the Celts were more dominant in western Europe, they did wield considerable influence in eastern Europe. The Scordisci, for example, while rather small in number, possessed a strong hold over the more numerous Thracian, Pannonian, and Illyrian ethnic groups, and were known to Alexander the Great. There is also a reference of a group of ambassadors from that tribe whom met the future conqueror of Persia in person. And in the wake of the Celtic invasion of Greece and Macedonia, one group would settle in what is now Bulgaria, and founded a short-lived kingdom which was centred around a city called Tylis. Others would travel across the Hellespont at the invitaion of the Bithynians, and later settle in the territory in central Anatolia called Galatia.
Anther possibility to explore is to create a successful outcome for the Suebi invasion and occupation of the Gallic territories of the Aedui and the Sequani, which occured during the First Century BCE, just prior to Julius Caesar's Proconsulship of Transalpine Gaul. The man that led the Suebi into Gaul at the time, known in the Latin sources as "Ariovistus", was said to have been a competent commander in his own right.
Or maybe some fifty years earlier, the Cimbri-Teuton alliance, that succeeded in toppeling the Romans in every engagement until the reformed Legions of Consul Gaius Marius settled the score at the battles of Vercallae and Aquae Sextiae. Maybe have them settle in southern or central Gaul.