1) A weaker Roman Republic would do have little to none direct influence on the "germanic" tribes, since there was practically no direct contact between Rome and the regions north of the Alps, safe for some long distance trade for amber, which was largely done by proxy.
2) The region of the Alps and todays south Germany and Austria was under heavy celtic influence at least up to the 1st CE AD. "Germanic" tribes settled further north and east and didn't start to migrate south and west until the 1st CE BC. (see: Cimbri and Teutones)
3) Germany was thinly settled and poor. There probably wouldn't have been enough people to conquer a densely populated and rich region like Gallia Cisalpina or northern Italy.
4) All "barbarians" strong enough to establish control over former roman allies would have been celtic in speech, culture, armarment, etc., not germanic.
5) The Romans had no concept about "germanic tribes" in this point of history. All barbarians coming from the north were "gallic", with maybe the odd rumour about some even wilder people settling even further north.
Sorry about the rant, but I learned pretty much about Rome's role in Germany, the origin of the germanic tribes, etc. last year at the uni. And I guess it's just my nature to share my knowledge...

2) The region of the Alps and todays south Germany and Austria was under heavy celtic influence at least up to the 1st CE AD. "Germanic" tribes settled further north and east and didn't start to migrate south and west until the 1st CE BC. (see: Cimbri and Teutones)
3) Germany was thinly settled and poor. There probably wouldn't have been enough people to conquer a densely populated and rich region like Gallia Cisalpina or northern Italy.
4) All "barbarians" strong enough to establish control over former roman allies would have been celtic in speech, culture, armarment, etc., not germanic.
5) The Romans had no concept about "germanic tribes" in this point of history. All barbarians coming from the north were "gallic", with maybe the odd rumour about some even wilder people settling even further north.
Sorry about the rant, but I learned pretty much about Rome's role in Germany, the origin of the germanic tribes, etc. last year at the uni. And I guess it's just my nature to share my knowledge...