This is an interesting scenario that I'm surprised isn't brought up more often - Alexander dying earlier is occasionally brought up, but it's always on the Granicus or even just prior to Issus rather than here. I've thought a little bit about making one of those short "timeline in a day" things on it but ultimately I'm far too easily stagnated with writing. Usually I have a rule not to pose what ifs to the forum on timelines I consider making, but now I'm going to break that rule. So here I am with a lot of ideas and an interest in discussing the scenario, and maybe I'll end up writing something after all.
Anyways, background on the scenario: it's 325 BC, and Alexander is fighting the Mallians after he was forced to return west after the army mutinied. He attacks the Mallian capital of Multan, and, recklessly, he gets himself shot by an arrow while jumping out to the forefront of the battle. The wound most likely pierced Alexander's lung. This was the closest Alexander got to death before he actually did die IOTL, and it's pretty clear that the wound significantly weakened him from there on after (Alexander is never again mentioned as walking or riding any long distance, instead taking a chariot or boat, and planned to ride with the fleet for the Arabian campaign).
The change I will make is simple: that arrow instead goes through Alexander's throat instead of piercing his lung, killing him instantly. No time to name a successor, no time for anything.
The thing I think is interesting about this scenario is that Hephaestion is alive and obviously in the "second-in-command" role (although he had yet to be named chiliarch), but hasn't been declared Alexander's successor. This differentiates the scenario substantially from the Granicus death POD because Philip's old guard besides Antipater guarding Macedonia has been weeded out by this point, and is substantially different from OTL because it adds Hephaestion to the first gen of the Diadochi and has the army stranded in hostile territory rather than in friendly-ish confines in Babylon, adding a certain Xenophon's Anabasis-esque feel to it. The other thing is that it would have been far too hard for the generals to maintain the image of Alexander being alive to the soldiers and to their enemies, so things will start unravelling very quickly.
Now, at this point, there are a couple important things to keep in mind with regards to the immediate succession. First, is that Arrhidaeus is with the army and there is no pregnant Roxane, meaning that Arrhidaeus should be acclaimed the next king pretty quickly. Heracles, Alexander's rumored bastard, had been born but there's no reason why he would be supported over Arrhidaeus, the main reason he was brought up as a possibility in real life was because Nearchus had been married into that family at the Susa Weddings, which haven't happened yet (and Nearchus had been sent out anyways IIRC). Second, is that a large part of the army and it's high command would not immediately be present: Ptolemy, Craterus, and Hephaestion were each sent out beforehand on various missions with large contingents.
Alexander on his return from India and this injury and then Gedrosia faced a lot of immediate challenges to his authority throughout his empire. Robin Lane Fox listed of some of the revolts/suspect personalities Alexander dealt with during the post-Indian campaign period IOTL, and I'll summarize that list:
- Greek mercenaries in Bactria rise up and name an Athenian their king
- Mercenaries in India murder the satrap Philip
- Tribes in the Hindu Kush rebel (quelled by Oxyartes, Roxane's father)
- Helmand Valley overrun by Iranians after Macedonian satrap dies unexpectedly
- Orxines, a member of one of the lesser Achaemenid branches, becomes the satrap in Persia after the old satrap dies without Alexander's approval.
- Susa held by two Iranian men who were once loyal to Darius and considered to be opportunists
- Armenia and Cappadocia not pacified (Alexander never subjugated either region IOTL)
- The Phrygians rebelled and probably killed their satrap
- The Thracians had yet to be truly pacified
In addition to this list, we can add two very powerful entities that might be inclined to take advantage of the opportunity Alexander dying provides, one which did "rebel" against Alexander IOTL, the other which may very well have had things went differently. These two men are Cleomenes of Naucratis and Harpalus. Harpalus, a boyhood friend of Alexander's that was lame and so was of no use militarily, had been made the royal treasurer in Babylon. He had control of at the very least the majority of the empire's wealth, and IOTL, he was not afraid to use it to his own advantage. He ended up spending a great deal of money trying to make Athenian prostitutes love him. Vainly he started wanting people to pay him and his lover the homage of proskynesis, which is obviously a direct assault at Alexander himself, implying that he was a god/king figure himself. Most likely he assumed that Alexander would die in India and he would never have to pay for this, but of course in real life Alexander returned and Harpalus fled in a ridiculous effort that ended up seeing Harpalus backstabbed by his own mercenaries and his stolen loot financing in part the Athenian rebellion against Antipater and a mercenary venture into Cyrene. Cleomenes, meanwhile, was technically just a nomarch but in practice the satrap of Egypt, being the designated middleman in tax collection from the rest of the nomarchs in Egypt to Alexander (as Egypt had no satrap), and also being the man primarily responsible for building Alexandria. Alexander considered removing him IOTL for his incredibly brutal methods to get gold, but gave him a pass after Hephaestion died, in exchange for him building a monument to Hephaestion IIRC.
Then there's also Athens, which I would imagine would be very eager to revolt against the Macedonians once they learn that Alexander is dead. However, the timing might not be as good as it was IOTL - they don't have the extra gold from Harpalus nor the influx of mercenaries looking for work (since Alexander hadn't forced the satraps to disband their armies yet).
So, clearly, there are a lot of obstacles for Alexander's generals to overcome to even return home, let alone maintain order and stability throughout Alexander's massive dominions.
That all said, here are the main questions I'm looking for, plus more thoughts in hopefully starting to answer them:
- What is the nature of Arrhidaeus' regency? Arrhidaeus will likely get acclaimed by the army as king immediately with little pushback from the generals, but how do the generals decide to govern the kingdom? Is Hephaestion accepted as Arrhidaeus' sole regent, being the obvious second-in-command to Alexander and seen by many as his spiritual heir (especially by Hephaestion himself)? Or do the generals come to some kind of pact between themselves agreeing to rule collaboratively? My hunch is that, since Hephaestion hasn't been made chiliarch, isn't present, and had a lot of enemies within Alexander's inner circle, he won't be allowed to become Arrhidaeus' sole regent, and the rest of the generals will agree to some kind of ruling council with Hephaestion as a sort of pariah entity.
- Darius III's brother, Oxyathres, was alive and with the army at Multan - what do the Macedonians do with him, and does he have any chance at staking his own claim at the Persian throne?
- How does the army and it's generals feel with regards to keeping the empire in tact? Do they purely just want to get the hell out of India and return home? Do they want to preserve Alexander's empire to the best of their abilities? Or somewhere in-between? They would return west, but under what purpose? And how easily could they transition into mercenaries in a more chaotic world?
- When the army returns west, does it do so the way Alexander did IOTL? Which is to say that the majority of it goes through Gedrosia while the veterans pacify the easier Helmand Valley. If it doesn't go through Gedrosia, how does the army get back?
- How good of a chance does Athens really have at overthrowing the Macedonians at this point? Am I right in thinking the Athenians would be in a worse position? Or are their odds about the same or even better than OTL's Lamian War?
- Can the Persians rally under a new king with enough time for them to effectively fight the Macedonians? And who would that king by? Oxyathres? Orxines? Or do you think that the Persians would remain loyal? (seems unlikely to me but is a possibility)
- There's also Antipater and Antigonus. Both were loyal to Alexander IOTL, but with the army a world away, and them being powerful, capable, ambitious men, do either of them go so far as to consider themselves independent actors from whatever Arrhidaeus' regime looks like? Do they work together in putting down Athens and the Anatolian -tribes? How much do they (or can they) assist the army returning from India?
- What roles do Harpalus and Cleomenes of Naucratis play? I can see both declaring themselves kings, and certainly both have enough money to support armies to protect those claims. How far can they go?
So I guess from here I'll turn it over to you guys: what are your thoughts on my questions and my analysis of the situation? I hope that I didn't kill discussion in the crib by sharing so much of my thoughts on the topic.

Just I've been thinking about this scenario on and off for a while and have plenty of ideas that I've been itching to share but have restrained myself from doing so due to a lack of finished product.

Would anyone be interested in a timeline on this?