Let's say that by some accident, the Romans around 30-20 B.C. (at the start of Augustus' reign) invents Black Powder. While the ingredient of saltpetre might be tricky, I don't think it's that improbable for gunpowder to have been created by chance.
Imo, Rome in this period is in a pretty interesting situation.
1.) Rome already has a large, professional standing army--a very important component in the successful deployment of firearms.
2.) Rome has an industrial base that can support the creation and the maintenance of firearms, as compared to the relatively disorganized and independent tribes in Germany and Dacia. This would create a decent advantage for the Romans.
3.) These developments would probably be enough to butterfly away the disaster of Teutoburg Forrest. While this will not have a significant change to the military, it will certainly have an impact on the expansionism of Rome across the Rhine.
So, discuss