Oh boy. Finally, an outlet for the unhealthy amount of thought I've put into this exact idea. Buckle up everyone, this is gonna be a long-winded one. Let's get to it.
but let's say he's healthy I just watched A video recently where someone said Julius Caesar would be able to subdue Dacia within 8 months and it made me laugh he could be stuck there three years even if Burebista is still assassinated
While 8 months certainly seems silly, I think there are two things to consider. First, the "conquest of Gaul", even though its completion took something like nine years, the latter two-thirds of that was spent mostly on consolidating what was already won. All of Gaul was more-or-less under Caesar's control by the time of his first crossing of the Rhine. He had established Rome as a power broker and peacekeeper (by intervening in the disputes between the Helvetii and Aedui and between the Suebi and the Sequani) and effectively stamped out those opposed to him (during his campaigns against the Nervii and Veneti), and by 56 BCE, he was effectively campaigning in Germany and Britain to protect what was already won, and any conflict with the Gauls thereafter took the form of securing control over the incongruous Gallic leadership. Note that although the largest of the Gallic revolts ended at Alesia, there were intermittent rebellions in Gaul as late as the reign of Vespasian, yet we do not say that the complete conquest of Gaul took an entire century because the bulk of the heavy lifting had been done before Alesia; and I would furthermore argue that the bulk of that work had been done by the time of Caesar's first invasion of Britain. Secondly, Caesar's war with Pharnaces was, at its outset, predicted to take a year at least. Pontus had been Rome's most formidable enemy since Carthage, and it was rightly thought that the threat posed by Pharnaces would tie down Caesar for a year or more. However, this war ended in something like five days. Did this have anything to do with Caesar's skill as a general? Probably not. The swift collapse of Pharnaces' position is probably more emblematic of how tenuous his own seizure of power was, and how it did not take much pressure to undo his brief attempt at regime consolidation. To apply this to Dacia, we have precious few sources that deal with the particulars of Burebista's regime, or how comparable the urbanization of Dacia was to that of Gaul (which is a crucial detail). Gaul was urbanized enough that it was easy for the Romans to take the reigns of power (like they did earlier in Africa, Macedon, Asia, and Syria), but not so centralized that they formed a coherent unified front against the invading Romans (like Macedon or Carthage had), which is probably why there was only one Gallic War, whereas there had been three Punic Wars, four Macedonian Wars, three Mithridatic Wars, etc. Whether or not the collapse of Dacia would be as rapid as that of Pharnaces, or whether consolidating control over Dacia would be as difficult as that of Gaul is impossible to say. My main point is that it's not too absurd to predict a short war of Dacian conquest, perhaps followed by a decade or more of consolidation, and the outcome probably has little to do with Caesar himself.
The Parthian Empire in there Second war against the Romans the Pompeian–Parthian invasion of 40 BC really didn't get anything out of it and Caesar was my much better General than Marc Anthony could Caesar avenge his friend Crassus death most likely but he is stuck there for quite a long time Cleopatra's most likely assisting with military assistance because Caesar gave her a Roman province already.
I have my doubts as to how much help Egypt would be able to provide in such a scenario. They had just recovered from a civil war, and Cleopatra's claim to power was perhaps not as secure as Caesar's following his civil war. A century or more of mismanagement by the Ptolemaic dynasty meant that Egypt was not approaching anything close to its full economic potential, and it would not do so until measures were taken by Augustus after the Battle of Actium. While it would certainly be better than nothing, I have doubts as to whether the contribution of Egypt would be any more decisive than the contribution of any other Roman ally like Galatia, for example.
An early conquest of Dacia and may even further expansion into Ukraine could be interesting-- not very defensible either, but having the lower Dnieper be Romance-speaking could influence post-Roman developments there.
However, Caesar is an extremely talented general, and he never showed any real signs of a deteriorating mind, but keep in mind that Roman knowledge of the Far East was limited to the far eastern bits of Mesopotamia (perhaps a bit of Central Iran) so there is a high chance that components of his army could be lead astray due to poor navigation.
Another thing to keep in mind for a supposed Roman campaign is that the Parthians won't simply keel over and die at the sight of oncoming Roman Legions. Say what you want about the Parthians being decentralized and loosely organized, they knew how to fight back against Rome. While Caesar may be able to beat them back and seize some cities in Mesopotamia, there could be multiple Surena-Like generals we weren't aware of at the time due to a lack of knowledge on Parthian internal politics that could've given Caesar and his minions absolute hell throughout the East.
It's also crucial to note that applying concepts like "defensible borders" to Roman strategic thinking by the time of Caesar is very anachronistic. This type of long-term strategic planning did not come about until the reign of Augustus at the earliest, and it was only possible because of the dynastic security of Augustus by his promotions of Agrippa, Tiberius, and Drusus in addition to his own careful and deliberate planning. Whether or not Caesar would be capable of carrying out the inglorious, slow, but necessary conquests of areas like Raetia, northern Spain, and Moesia that would be crucial to the defensibility of any Roman presence in Dacia is uncertain. We don't have a large enough sample of Caesar's own political ambitions to say for sure. We do know that he was capable of long-term planning with regard to Rome's provinces (his Julian law of extortion is a clear enough example of that), but whether or not he had the political foresight, or even capability, to carry out such inglorious wars is impossible to say. Something that we do know however, is that Caesar had a wonderful talent for propaganda. His two British expeditions were unmitigated disasters that gave him no substantial gain apart from their public relations value. He very nearly lost substantial portions of his army, and it's no small miracle that he was able to recover from these calamities at all. Claudius' later army mutinied before they even crossed the channel, and engaging in such far-reaching conquests may have been beyond Caesar's ability to lead his men. As an example, look at the disastrous effects that the mutinies of Alexander's army had on his own expedition, and compare this to the fact that Caesar had seen at least two major mutinies during the civil war.
I think the following is also crucial to consider: Caesar's wars thus far had not been against organized, powerful states with coherent governing regimes. The only comparable example was his war in Egypt, and that was almost a disaster before he was bailed out by Mithridates of Pergamum. His war against Pharnaces (who likewise led something more comparable to a modern state) was presaged by half a century of vigorous wars fought between Rome and Pontus in order to weaken their power to a degree that Caesar was able to swat them aside like he did. Looking again to the example of Carthage: it took numerous crushing victories by the Romans at the Metaurus, Carthago Nova, Zama, and numerous others in Spain before Carthage finally capitulated And even then, it took another half century and a three-year-long siege before that great power was finally humbled. Parthia was unlikely to capitulate any more easily, especially given that Rome was not yet clearly the dominant partner in that relationship (as they would come to be in the late 1st century CE). However, as I illustrated above, it would not be absolutely necessary to win any crushing victories over either the Dacians or the Parthians in order to achieve his domestic policy aims. His talent for propaganda may enable him to spin a marginal victory over or a strategic retreat by either power into the domestic leverage that always appeared to be after in the end (with conquests merely serving as means to this end).
But it should also be considered that Caesar's regime (and ultimately his life) were destroyed from within while he was still in Rome. Caesar's long absences from Rome generally saw his rivals and enemies gain considerable ground (whether this was during the Gallic Wars or his civil wars) or at least undermine the certainty of his own political security. Caesar's career almost ended in the standoff in 49 BCE, and even though he was unquestionably the most powerful man in Rome at the time of his death, it is far from certain whether or not an extended campaign far from Rome (much further from Rome than Gaul, which is an important detail) would be sufficient to undermine his own political support within the city itself. I could see it as being more feasible that Caesar would make periodic returns to Rome in order to reward his political allies and take measures to secure his own power (with continual grants of imperium or what have you) while he maintains a periodic correspondence with Rome as he sought to shore up his political support. However, the amount of traveling this entails would almost certainly have adverse effects on his health, and whether or not that would be sufficient to cut his life short is a relevant question.
Lastly of importance is, what were Caesar's plans for the succession? He clearly had his eye on the young Augustus by the time of his death, and some sources even claim that Augustus was a candidate for Caesar's magister equitum. Whether or not Caesar would have the time or foresight to consolidate the complex and indirect means of succession that Augustus later devised (with tribunician authority and personal grants of maius imperium) is possible, although it is important to note that Augustus took half a decade to secure this for himself and a further decade or two to do so for his heirs-apparent (at various times Agrippa, Gaius, and Tiberius). Augustus had the advantage of coming to power at such a young age that he was able to take his time forming the shape of his regime (which would later be called the principate), and whether or not Caesar's own foresight or political skill would allow this. The implication seems to be that a civil war is probable upon his death, although delaying his death by any length of time would only serve to strengthen the position of Augustus, and so the eventual long-term outcome may be similar to IOTL.
