The battle in the teutoburg forest.

It is merely speculation but i think that would be enough for the roman empire to survive, at least for a couple of centuries.
A great roman victory would probably have secured the province germania to them. The romanization process already had begun. The first new roman towns had be established before 9 ad.

Many important tribes that later destabilised the empire and contributed to it`s fall, would never have existed. The franks, the Alamanni, the Lombards, the saxons, the amrcomani. They all lived in or around planned provinces.