Caesar was an old man, when he became dictator, and there are some hints, that his health was already not the best.
As already mentioned, the parthian empire is far away and communication is an issue these times. Caesar can beat the parthians and revenge Crassus pretty fast. Actually Augustus did this too, but much more clever with negotiations and a lot of propaganda after he got the eagles back.
Conquering the parthian empire is a fully different challenge. Mesopotamia alone is not enough, as Trajan and Hadrian recognized the hard way. Actually Mesopotamia was almost lost again, when Trajan moved home and died. You need to conquer at least Media and Persis too, in order to deprive the parthian empire from its economic base. And this will become a very long war. And without proscriptions, there are still a lot of republican aristocrats alive in Rome. And some of them command legions.
Sometimes I think that Caesar tried to escape from Rome with this campaign. As Augustus recognized clearly, after a won civil war, the reconstitution of the republic was the most important task. Obviously Caesar shied away from this challenge or underestimated it dramatically.
Augustus was a young man, and he was lucky to survive and grow very old, even if his health was never the best. He was already near to death in 23 BC. It would be an interesting POD, if he dies and Agrippa with young Marcellus, Tiberius and Drusus have to consolidate the empire. Marcellus dies shortly later by natural causes. So there is just Agrippa , who is not a native member of the high aristocracy, and a bunch of roman aristocrats.
Sorry but I disagree again. I know many writers have made theories about Caesar's psychology. All based on the idea that Caesar was following chimeras. But all did nothing but inventing their Caesar of romance.
What is puzzling is that they imagine that when Caesar and Augustus did the same thing, they had opposite purposes.
Caesar did not shy away from Rome. He went for emergencies. There was an emergency in the east. Crassus' disaster at Carrhae and the civil war had dangerously weakened Rome's positions in the east. There was a strategic interest for Rome to reestablish its hold in the east. That's exactly why Anthony, who was the dominant player in the first years of the triumvirate, fought parthians (in the first years through his legate Ventidius Bassus who won the campaign).
Besides, after the civil war, it was a good solution to go for a foreign military campaign if a ruler wanted to heal the wounds of a civil war and if he also wanted not to have his rule antagonize too much people who did not support his regime.
Caesar planned a 3 years campaign in 44. There is absolutely no evidence that he was planning some kind of Alexandrian grand campaign.
In 27, just after he had been granted the title of Augustus, Octavian went for a 3 years campaign in Spain. Almost the same thing.
This an obvious parallel.
Caesar did not either underestimate the difficulties of reestablishing a republic. He said that he knew he was hated by many aristocrats. But he also successfully brought the most ancient and prestigious patrician families to work with him : the last Fabii Maximi, the Aemilii Lepidi, the Valerii and the Claudii, the Servilii (who were some kind of false plebeians) and some Cornelii.
Concerning his health, one can't know. But Augustus had a rather poor health, which did not prevent him from reaching almost the age of 77. Caesar could have died quite quickly and there would have been the problem of succession. But he could also have lived 20 more years.