The list of barbarian lands the Romans conquered is actually pretty lengthy. The moti9vation in almost all cases was internal, though, so it would also have to be in this case. There could be all manner of good things top be had, but that alone won't trigger the invasion. Rome could live well with neighbours that had things it wanted. they could trade. But a situation in which internal dynamics makes the conquest desireable will do it. the problem is, after Augustus, conquest really isn't worthwhile any more. You note that aggressive expansion afterwards is rare, not because it couldn't be done - for Germany, Mesopotamia and Arabia I'll give you Britain, Arabia, Mauretania, the Dodecaschoenos and Dacia as examples where it worked - but because it had great risks and few rewards. Felix didn't get suicided because he lost, he had to die because he almost won. The new reality simply hadn't caught up with his mindset yet. So a prolonged Late Republican crisis might do it, provided we can prevent the second round of civil wars from destroying the whole state. After all, Augustus stopped trying after a single deefeat because he could afford to. If he or Tiberius had been facing adversaries in a competitive senatorial aristocracy, they would probably have tried again, and quite plausibly succeeded. Or someone else with embition would have.
The problem, I think, is that we are also looking at Roman intention with hindsight. If there was ever a moment when Rome stopped considering Germany a viable target of conquest, that would be under Hadrian, not Augustus. There wasn't really a concept of 'here Roman, there barbarian wilderness' at that time. The Empire for a long time after the great waves of conquest in the first century BC remained conceptually a mosaic of states and statelets, territories and colonies. To a Roman in the reign of Tiberius or Caligula, the territory of the Batavi, Treveri or Ubii wsa not conceptually very different from that of the Frisones or Chatti. Vassalised barbarians, subject to Rome, all of them. Some answered directly to the authority of a promagistrate, others didn't, but the idea that a province was a designated territory and had a clear 'outside' needed a few more decades to really take root.
I'm pretty sure that if we stay close to OITL, the one thing that would have motivated Rome to conquer Germany was an easier time of it. It was just given up as not worth the trouble. If Rome had stuck to its old political dynamic longer, Germany would sooner or later have been invaded, too, because it was there and someone needed a triumph. Biut in that case there's also a good chance - better than even chance IMO - that Rome would have destroyed itself before that happened.