@General Zod: Er, the Indus? All the way to India? That's a pretty big territory. Of course, Trajan wanted it, but just made it to the Persian Gulf.
Yep, it is reasonable to assume that with a Romanized Germania-Bohemia-Dacia and Caledonia, the Empire has the manpower and resources, both the ones spared from the shorter, safer borders, and from the extra provinces, to allow Trajan to subdue Persia, too, which ends the Parthian Empire as an organized force (Alexander and the early Caliphs conquered Persia, Trajan beaten it severely, ancient Persians were not invincibl, it isi reasonable to assume that with more troops and money and less problematic otherborders, he would have finished the job) and turns it to a mid-term anti-guerrilla operation. Something the Romans were rather good at, and by now they dod not have any major border enemy left. If they can deliver Parthia one complete knockdown in thse conditions, they will never leave.
Expanding into India, of course, would be another matter entirely.
Caledonia could've been conquered - under Domitian, general Agricola could've done it, but the emperor envied him and forced him to commit suicide.
If they own Germania, Marcomannia, and Dacia since August-Tiberius, and Britannia since Claudius, conquering Caledonia looks more like an ongoing cycle of gradual expansion, and more of a mop-up operation. It is not unreasonable to butterfly away this event.
But the problem is: Could Germania, Bohemia and Caledonia really pay that much of taxes? Until better ploughshares were invented, the soil was too heavy for agriculture. These provinces could turn into money sinks.
Never money sinks, only less profitable thanks to the shorter border and since some agriculture was always possible even with old ploughshares. Anyway, since better ploughshares were relatively quickly invented during the first true urban-agricoltural settlement of Germany in the Dark Ages, it is quite reasonable to assume they would have been even much more quickly during the first Romanization settlement of Germania. We ought to assume that barring clear contrary evidence, whichever technological advancements did occur in Europe during the Dark Ages collapse, it would have occurred anyway, and much more quickly, had the Roman Empire stayed toghether/expanded further.