I would have to believe that a Germanic incursion into Gaul is going to happen sooner rather than later if the Romans do not establish control up to the Rhine.
As I tried to point out in the aformentioned
thread, the whole "Germans are going to invade Gaul" is essentially a convenient pretext for invading the region, playing on the confusion between geographical names (Gaul, Germania delimited by Caesar himself on the Rhine, while nobody really made this distinction before).
You already had deep contacts between Celts and Germans at this point, would it be because these Rheinish Germans were at least heavily celtized up to names (Ariovist being a celtic name, for exemple). Not only in Belgium where trying to differenciate the Celtic and Germanic tribes is a cute attempt at best, but in different places in Gaul (as the Boii of Aquitaine : arguably, Boii were a celtic people in Germany; while Tarusates were an Aquitain people in Gaul. One more exemple of the weaknesses of Caesar geographical distribution).
While it's going to have an impact on Celtic confederations, as it would happen for the Great Invasions of the Late Antiquity, we'd be really far from "Barbarians ate my baby and destroyed my civilisation" trope. There's again, we have too groups with a similar culture and not aliens to each other.
Aroivist's confederation wasn't that distinct from what Aedui did (he even tried to ally himself to Rome, as the formers did), meaning structure on a relativly large scale more or less isolated peoples.
I'm not sure having a third or fourth confederation bordering Rome would just play into Romans hands. At least, it's highlighting the Roman influence (direct or indirect) on the structuration of gallic entities.
I do wonder however what the economic effects of less Gallic slaves to Rome is going to be in the time period of Caesar.
I think this would be partially counterbalanced by trade. As it happened with Germans IOTL after the Ist century BC, buying slaves at the source replaced the roman raids on the other side of Rhine or Danube.
It would probably be less impressive than Caesar's human loot, granted, but giving Italy was already saturated with slaves since half-of a century, it would have only a limitated impact, IMO.
The absence of Gallic auxiliaries could play a very interesting role in the ongoing Civil Wars if Caesar survives. He relied heavily on these (especially cavalery-wise) to crush his opponents.