I had Arminius unite them by force and made the process of solidifying them into an actual country take a long time, but I honestly think it came out a little bit ASB all the same. I have to admit, a unified Germania in antiquity is quite unlikely. A more successful Arminius was the only remotely realistic POD I could think of that might result in the tribes banding together for a long period of time, and that's still a stretch.
In my opinion, the question is that by Arminius time, Germania was not developed enough to sustain a stable unified polity for a long time. More advanced places such as Gallia and Dacia did not, and they had a larger and more sedentary population in comparison, more natural resources etc... Of course, you can always assume an unplausible, but still possile, amount of luck.
A more developed Germany IS possible, but it would more sedentary as well. There would need to be cities, not tribes. So no point in invading and settling Roman territory (not more than OTL at least).
I'd go as far as claiming that the "backward" political structure of the Germanics way a key factor in their favor when they defeated the Romans in the fifth century.