The Elbe may have been the ultimate aim of Augustan policy in Germania and this may have been because of a desire to shorten the northern frontier of the Empire. This theory has been around for some time, but since it is not attested in any of the sources (so using words like "undoubtedly" about it is highly misleading) it is far from universally held. More recently it has been called into question for some very sound reasons.

To begin with, what may be obvious to us looking at modern maps is not likely to have been obvious at all to someone of Augustus' time. Without the benefits of modern geographical knowledge, Roman conceptions of what lay beyond their borders were often rudimentary and the position of the upper reaches of the Elbe in relation to the Danube could well have been something rather beyond their knowledge. If the information in the famed map of Agrippa (as reported in later writers) is anything to go on, information on the lands east of the Rhine presented a rather distorted view of the world beyond the frontiers and any assumptions made by looking at modern maps of the same area are likely to be problematic.

Regardless of what Augustus' aims may have been (and there is quite a bit of doubt about this), the fact remains that in Varus' time all Roman forts, outposts, causeways and colonies had been established between the Rhine and the Weser. Dio indicates that Varus' brief when he was sent to Germania was consoldiation, not further expansion, and it was the tribes between the Rhine and the Weser which were to be his focus.

Some useful reading on Roman strategy, frontier policy and the limits of their geographical knowledge:

Austin, N.J.E. and N.B. Rankov, Exploratio: Military intelligence in the Roman world from the second Punic war to the battle of Adrianople (London 1995) 292p.
Dilke, O.A.W., Greek and Roman Maps (London 1985) 224p.
Luttwak, E.N., The Grand Strategy of the Roman Empire (Baltimore 1976) 255p.
Mann, J.C., 'The Frontiers of the Principate' in: ANRW II-1 (1975), 508-533.
Mann, J.C., 'Power, Force and the Frontiers of the Empire' in: JRS 69 (1979), 175-183.
Mattern, S.P., Rome and the Enemy: Imperial Strategy in the Principate (Berkeley 1999) 259p.
Millar, F., 'Emperors, Frontiers and Foreign Relations, 31 BC to AD 378' in: Britannia 13 (1982), 1-23.
Sherk, R.K., 'Roman Geographical Exploration and Military Maps' in: ANRW II-1 (1974), 534-562.

A particularly useful essay on the limitations Roman geography placed on commanders and the problems involved with suppositions by modern armchair generals armed with maps made from satellite photos is A.C. Bertrand "Stumbling Through Gaul: Maps, Intelligence, and Caesar’s Bellum Gallicum" The Ancient History Bulletin 11.4 (1997) 107-122.
Cheers,

Thiu