What's your PoD? Germanic civilisation would inevitably have Roman influences as much as they had Celtic influences. I think the best opportunity is to have Arminius not only smash Varus at the Teutoburg Forest, but then also have Arminius defeat Germanicus, who OTL gained his name by avenging the defeat of Varus by recovering the legionary eagles in his more or less successful campaigns against Arminius. I like Idistaviso in 16 AD as a PoD. The very name of the place suggests a religious element, and if Arminius can once again defeat the Romans (perhaps by a raid on Germanicus's camp, killing him and plunging the Roman army into chaos), he'll gain serious prestige both amongst his own people and among the Romans. Certainly, when he dies his coalition will fragment and be subject to Roman rule, but Rome won't quite be as powerful beyond the Rhine as OTL, and Germanic peoples will have an inspiration to look toward. OTL, some have cited Arminius as the inspiration of Sigurd/Siegfried, legendary figures, in part based on the names of his relatives, many of whom had the "Sig-" element in their name (like his father "Segimer"). Arminius's name was likely given to him by the Romans, instead of Germanic folk etymologies like "Hermann" or something cognate with the first element of religious terms like "Jormungand" or "Irminsul", like has been suggested at times.
Basically, Arminius ends up as a demigod, and the Romans support him and end up creating some nice puppet states after he dies. But it gives Germanic civilisation more room to breath.