Ye, the Great Illyrian Revolt is a good POD, no doubt.
But if there had been no lost Teutoburg battle - Germany would have stayed Roman. With or without Illyrian Revolt.
Even if there had been no Illyrian Revolt - Augustus Octavianus would have left Germany after loosing a few legions. He was that kind of a guy, he was not a risky type, he was not ready to bet an army after the army in a risky game of conquest.
It's a famous saying of his how stupid it is to fish using a gold angle - the possible profit of a fish is worthless comparing with the possible loss of gold.
Yes, the illyrian revolt is perhaps the best POD. It is more historical and less arbitrary, than Tiberius dies and Germanicus becomes emperor. But therefore we must discuss the reasons for the illyrian revolt and proper measures, how to avoid them. I am afraid these measures become a bit complex and have nothing to do with military.
Without the illyrian revolt, Tiberius and Saturninus would have invaded "Marcomannia", todays Czechia. Let's assume Marbod and the allied Elbgermans would not beat the crap out of 12 legions. And even worse, the germans in the province Germania Magna would not jump at the chance to revolt already in 7 AD, while the romans are busy with Marbod. Let's assume further, that the Dacians are on Augustus list, too. Actually they caused a lot of trouble and already Julius Caesar planned to attack them. In this scenario in the first half of the 1st century AD we will end with a roman border from the Elbe, via the czechian and slowakian mountains, via the Carpates, down to the Danube Delta.
Historians dispute heavily, if Augustus had a plan at all. Some argue, that his plan was to protect the roman provinces, especially Gallia and Italy. Well, in this case the border above and a plan to reach this boarder would be reasonable.
But how would this change the overall situation?
- the geography of Germania is still difficult and more costly for logistics
- and perfect for assymetric warfare
- administration and romanization is more difficult and costly than elsewhere, because the german nobles are socially very different compared to the mediterrenean or gallic nobles.
- the ressources are still not sufficient to cover the costs of romanization and security. It could take centuries like in Britain to change this negative Return on Investement.
- the ancient economy of the roman empire was worse than modern people may think. The taxable surplusses as well as economical growth was rather limited. Therefore a lot of roman emperors have been very busy to fight against bankruptcy. Not talking about the 3rd century, when the empire came under pressure.
- beyond these new borders we have even more barbaric germans. And the roman neighbours will start to upgrade and arm them via unintentional development aid, as they always did in OTL with the tribes along their Rhine-Danube border.
Why should we assume, the germans will not revolt?
Why should we assume, that an emperor like Germanicus can not read the breviarum and do some easy calculations? The breviarium was a small book, Augustus handed down to Tiberius. It included most propably lists of the imperial assets, income and costs.
What exactly would be the significant change in the roman way of provincialisation, in order to have a chance to hold this area at all?
Why should we assume, the romans would not be back to the Rhine-Danube border a few decades later?