Didn’t Arminius have a plan to unify the Germanic tribes between the Rhine and Elbe as a warlord? If he succeeds in rallying the tribes to a cause, he has to specify what sort of cause that is — given that Roman soldiers were raiding and sticking oars into Germania, they could come to the conclusion that it would be making it so that Rome can no longer interfere in German affairs, but that would take a substantial weakening of Rome’s offensive capability. Conclusion: an invasion of Gaul, not considering Arminius’ odds.
The Cherusci-led German tribal league would only be able to properly invade Gaul and succeed under specific circumstances, which might require a few spontaneous rebellions happening all at once. If Germanicus dies early, Rome falls to political instability after the death of Augustus, and the Illyrian revolt drags on, only then could Arminius have a chance at invading Gaul and inciting a local Celtic rebellion, imo.