Could Rome have assimilated the Germans like they did the Gauls?
Perhaps Varus could have been made governor of the province, but a more competant man put in charge of the invasion.
How far would the Empire have gotten? The Elbe, the Vistula?
If, hypothetically, Rome manages to conquer and thoroughly Romanize the Germans, would Rome have fared better during the migration period? Without hostile Germans being pushed by the nomads into Roman territory, and the Roman-Germans as a group loyal to Rome itself rather than the situation of OTL's foederati which produced Ricimers, Alarics, and Gundobads, perhaps the legions could have put up a more solid defence against the Vandals and Huns, and avoided both the social and military problems the barbarian Germans created.