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Considering Caesar's track record in Gaul, Germania with its tribes should be less of a problem (Divide et Impera, anyone

). The Gauls were arguably higher developed as they were beginning to form embryonic states and cities. Who knows what another century or two would have done to them if left alone. Germania should be less of a problem IMHO, considering that you can fumble around with Germanian history since no written sources from Germania itself exist. That makes you less reliant on facts and gives you more freedom to have Caesar squash Germania. Caesar could ver well establish a border on the Oder or even move on to the Vistula and Carpathians which gives Rome a large population, but a much shorter border to defend. Romanwank! Maybe Hibernia and Scotland (once Brittannia is set up) could be added too since it removes some more borders, and don't forget Denmark!
Of course there needs to be an incentive to conquer Germania. For Caesar it could be glory, but I recommend introducing the heavy plough early to make cultivation of Germania easier, giving Rome an economic incentive to take it. It's not as if the Romans couldn't invent it. They just need to put 2 and 2 together.
Hope my ramblings helped.
Sorry, but Divide et Imperia wouldn't work the way it did in Gallia. Also, Conquering Germania would be of little use to Caesar.
1) As you mentioned, most of Germania lacked the proto-cities, oppida, which where quite common in Gallia. The "germanic" tribes were less strictly organized, hell, they only started to develop a strict social hierarchy and organisation in the centuries after Caesar had conquered Gallia and the Roman contact and trade played a major role in this development.
While this made "conquering" Germania easy (the Romans considered it conquered before Tiberius abandoned it, which was a wise decision imo), it made it very hard to rule it. In Gallia, the Romans could use their usual approach of divide et imperia. They allied with some of the most important nobles, took their sons as political hostages, educated them and turned them in to "civilized", roman citizens, which stabilized the Roman rule and was a major part of the romanification effort.
This approach was used in Germania, too. After Augustus declared Germania conquered, they tried to turn it into two roman provinces by pretty much the same methods used all over the empire. Work through local elites, grant cities a lot of rights, urbanize and romanize the people, build infrastructure, etc. The problem was, the germanic tribes had in mostly not traditional elite of nobles. They were dirt poor, which made them pretty much all equal. Power was mostly gained through great deeds, like being a good warrior and an able warleader. Also, tribes were not stable. They would split up, swallow weaker tribes, flee from or join stronger tribes, etc. It's hard to urbanize and control such a fluid society in such a sparsely settled region as germania.
2) Germania was not worth taking. There were no metal mines, except for some important lead mines just east of the Rhine, which were taken either by Caesar of Augustus (not sure which and too lazy to look it up right now

) and were never surrendered afterwards. The rest was seen as a region with no important natural resources, very little usable farmland and with wet and cold weather.
3) Caesar himself had defined Germania as the lands east of the Rhine and Gallia as the lands west of it, so he could end the war after he had conquered wealthy Gallia and wouldn't had to continue the war into Germania, where very little would be to gain.
4) The Romans didn't know, that they would shorten their border by extending east. For them, the Rhine was the most sensible border. It was easy to guard, it made defending wealthy Gallia easy and east of it were only unorganized barbarians, which could be easily kept at bay by a military expedition every once in a while or by supporting inter germanic wars.
Edit: sorry 'bout that, but the whole "Rome should have better conquered Germania" cliché just rubs me the wrong way.
