but do you think that it would be profitable enough to A) keep the Romans there plus add nessarcy legions to garrision the Province? and B) raise even extra legions for further expansion?
That´s difficult to answer.
Not losing 3 legions but only a few thousand soldiers is definitely a net gain for the Romans. It´s quite easy to imagine that in this case treason by Germanic tribes will led them to spend even more effort on Germania Magna. A matter of honor so to speak.
Not winning the battle at Teutoburg Forest is definitely a net loss for the alliance of Germanic tribes. Difficult to see how the alliance would survive that.
If the Romans are smart, they´ll accept the submission of Germanic tribes. However Roman pride might demand that they punish every tribe involved in the Teutoburg Forest battles?
So in one scenario they might divide their opponents, in another they´ll force them into a fight to the death. Which is more realistic, I just don´t know?
In a best case scenario they´ll divide their opponents. Some local revolts but they could occupy Germania Magna up to the river Elbe. Inside 10-20 years they would discover and start to exploit some of the rich mineral deposits in Germania Magna. Will they stay long enough to discover them?
That wouldn´t be enough to pay the 7-8 legions normally stationed at the river Rhine and upper river Danube border. But it would shift the burden gradually.
With several decades of peace they´d get a shorter border - perhaps needing 5-6 legions only? - plus mineral resources (gold, silver, copper) to pay perhaps 2-3 of them? Rising gradually. Meaning that "older" provinces won´t have to pay for them.
If it works perfectly well, the Romans might be able to raise 2-3 additional legions later in the first century AD. Given that Germania Magna now is able to pay for several legions.
In that case I´d think that Roman Emperors might be interested in Britain now. Tin, you know. You want to mint bronze coins, the Roman As, you need copper (available in the Roman Empire) and tin. The best source for now is in Britain?
There´s also tin in the Ore Mountains of course but not known yet.
And I should also caution that the river Elbe isn´t that big of an obstacle compared to the river Rhine. The river Elbe is / was wide and shallow. Lots of places where warriors could wade across the river. So it´s still possible that a weakened Roman Empire might retreat from the river Elbe to the river Rhine in the 3th or 4th century.
On the other hand a weakened Roman Empire giving up so much territory might get enough time to fortify the Rhine border? Using Germania Magna as a buffer zone, so to speak?