The blow of theTeutoburger Wald battle was mostly psychological and political, not military (losing three legions hurts, but during the Civil Wars these casualties would simply have been taken and replaced). A better policy, better generalship, better counterintelligence, or simply more luck could have averted it. I keep wondering how much morale and psychology played a role in the defeat - the Romans have a record, in similar situations, of fighting like cats in a bag, and here they were simply slaughtered. Was it facing their own troops that did it, or might they have stood the night if the cavalry had held out?
There's an ATL in 'What If?' that expands on this question, assuming that Roman rule and civilisation stretches throughout the Germanic world. That would make Germany into another version of Gaul, heavily Romanised and prosperous. I am not sure it would happen - the role the Germanic threat played in the Romanisation of Gaul would have to be taken into account - but if it did, I don't think things would stop there. Northern Europe might not look hospitable to Romans, but I'm pretty sure Gauls, Illyrians, Britons, Thracians and Dacians would not see a problem with seeking opportunities there. Contrary to widespread opinion the Romans (or rather the Gauls) could work heavy soils, so the valleys of Germany might be turned into prime farmland dotted with villas. Also, there are considerable silver, iron, and coal deposits to exploit. In short, it might create another Romance country on the map of Europe, placing the cultural border somewhere east of the Elbe (though North Germany is likely to be somewhat left out in the cold)
A question that keeps bugging me: the Romans would have loved the shallow, sheltered, nearly tideless Baltic. Would they have gone exploring and trading? Maybe the natural harbour of Old Lübeck would be developed into a jumping-off point for cutting out the middleman in the amber trade. And the import of reindeer for the Colosseum
One thing is for sure: the Germans would have made great recruits for the auxiliaries.