Rome had the ability if circumstances had allowed it to conquer Germany, it would have been the biggest undertaking the empire would have ever done though.
Itd of required hundreds of thousands of men, dozens of legions and exceptional leaders to conquer, hold and romanize the new provinces, with the potential gains it simply wasnt worth that big an effort.
Besides Rome had good natural frontiers, the Rhine and the Danube why extend to a place were their wouldnt be such favourable natural defences?
I do think though that when Rome ceased conquering its decline was sealed, you cant have an empire that size forever it needs new energy it needs to expand and grow, when that ceased it was only a matter of time before the barbarians chose the right time to invade and tipped the roman empire into anarchy.
Perhaps Rome should have kept alive the dream of an ever expanding empire in order to A, keep the legions busy fighting wars rather than sitting around on frontier duty B, destroy major external threats and C, to avoid complacency.