Is it possible, however unlikely, for a megalomaniacal, but clever, Roman Emperor to go on massive conquests in the East with most of the Roman army, perhaps stopping only at China? In the aftermath, the giant Roman state would split into several usurped states like the Hellenistic states after the death of Alexander.
My guess that something like that was possible only during Late Republic when Rome had a lot of legions under arms and multitudes of ambitious politicians and generals.
Well, Parthia was a hard nut to crack and it is difficult to find a certain POD.
For example triumvir Crassus dies before the beginning of his unfortunate invasion into Mesopotamia. Or he gets seriously sick and makes his son Publius Crassus his successor - the old man lives long enough to get the approval of Julius Caesar and Pompey. Which would not be too hard at least in case of Caesar, as Publius Crassus served under him during Gaullic wars and they went along just fine.
Publius Crassus was about 30 years old, so he was more mature than Alexander the Great when he started his conquest of the East.
In other respects Publius was similar to Alexander - he learned a lot serving under great general, had a successful experience of independent military command. He even was famous for his personal courage like Alexander the Great; he fought on a horse when he met his glorious death.
With good luck he could conquer the Parthians and recreate something like the Seleucid Empire, which was not altogether forgotten in these lands.
Why not? The Macedonians/Greeks somehow managed to hold these lands for a few hundred years.
But the Romans had to do something to improve their cavalry and missile troops. Better both...