The reason I'm always sceptical about Germania/Persia conquests is the simple fact that Rome was an Empire based first and foremost around the Mediterranean Sea, not the European continent. All of Rome's conquests down to about 80BC were of areas immediately bordering the Mediterranean, and, Gaul aside, the conquests of Illyria, Pannonia and the eastern Balkans were chiefly a search for a more defensible frontier. Roman expansion stopped at the Rhine because the Rhine can be supplied from the Mediterranean- similarly, it stopped in northern Iraq and the Armenian highlands because these were about as far away from the Mediterranean and Black Seas as Roman logistics could manage.
Conversely, I do think that this makes the conquest of at least the Arabian coastlines, and perhaps the Crimea in its entirety, a strong possibility for a Romano-wank: both of these areas can be easily tied into the Imperial centre through shipping. Caledonia and Hibernia are also not quite out of the question for this reason, though unlike Arabia and like Germania, they suffered the significant disadvantage of lacking anything that the Roman Empire, which was an agricultural state that relied on tax, particuarly wanted.