The descriptions of him planning the conquest of Arabia, Rome, and Carthage are fanciful speculations from authors writing hundreds of years after the fact, and have little historical validity.
Well, Krateros used the fleet he was building in Kilikia for the Qarthadast campaign to help crush the Hellenic revolt of the Lamian War. IIRC it was this navy that formed the basis of the Successor navies that clashed around Salamis-in-Kypros and in the Aigion; it was certainly sizable enough. There isn't much data on Nearkhos' similar expedition to conquer Arabia (and that one I have doubts about).
Nanocyborgasm said:
Even if Alexander planned any of these campaigns, it is unlikely he would've been able to carry them out.
He didn't plan to do it himself; Krateros was the man for the Qarthadast job as mentioned, with Polyperkhon as his number two. And at this point, Qarthadast was incredibly weak. The Syrakousans were able to actually
land in Africa and conduct campaigns before the conclusion of the Agathoklean wars. Qarthadast's navy was weak, and their army didn't do terribly well either.
While the establishment of a viable and long-lasting Hellenistic hegemony over Qarthadast is somewhat unlikely, Krateros had the resources and the skill to utterly trash them, far worse than anyone else did. And possibly conquer them.
Nanocyborgasm said:
He had to end his conquest of India after his troops mutinied, so at the very least, he'd have to wait several years before attempting any further campaigns.
He had already waited several years by the time he died. Hence why expeditions were viable. Also: the Makedonian army he brought with him to India wasn't by any means the only manpower in the Empire available or even the only trained Makedonian army available. I mean, honestly. There is not a significant manpower shortage in the Alexandrine Empire if said Empire has more than enough Makedonian soldiers to fight several vast wars over the course of the next two decades with virtually no pausing and large numbers of casualties.
Nanocyborgasm said:
Another problem plauging the new empire was cohesion between the numerous ethnic groups. Alexander planned to mix the cultures of the Near East and the Greek by introducing certain customs to his Greek and Macedonian comrades. In nearly all cases, these were met with resistance and outrage. One custom, called proskynesis, involved the prostration and kissing of a subject to the king, an act considered by Greeks contemptible and fitting only for worship of the gods. Alexander also married off many of his soldiers to Persian women, and himself married a Bactrian princess, Roxane, in his effort to create a kind of ethnic homogeneity. It would've surely taken years, possibly his entire reign, for these measures to take hold and only after much resistance from both the Greeks and the Persians, and Hellenism would have a far more "Eastern" flavor than it did in the Hellenistic Age.
Okay...but with the "Eastern" flavor would also come probably a longer-lasting cultural synthesis. Which when you consider the original longevity of Hellenistic culture in the Middle East, is pretty awesome. Frankly, I don't see Alexander's successors - if the Empire holds together - retaining these policies anyway.
Nanocyborgasm said:
As far as foreign intervention is concerned, I suspect that much of Alexander's later years would be spent more defending his domain than conquering other lands. The Mauryan Empire would present his first challenge, which would likely end in a compromise, as in real history.
Alexandros >>>>> Seleukos.

Besides, Seleukos made that compromise because he had better thing to do back west with the rest of the Successors. Alexandros, with far fewer if any such problems, will be less inclined to be bought off with a bunch of shiny elephant toys, especially when he already has his own. While any further conquest in India ranges from highly unlikely to impossible for Alexandros, I don't think that he personally would lose the Indos. And after that, hell, the Hellenic satrapies like Baktria will be plenty powerful with the extra Hellenic settlement.
Nanocyborgasm said:
The Parthians would probably consume a great amount of resources, and I suspect that with a unified empire, they would not be able to carve out their empire as in real life.
Yeah, but the Pahlava come after Alexandros' death. Other than that, yeah, they'd probably carve out their empire.
Nanocyborgasm said:
The Romans and Carthaginians would likely be left to their own devices, as the empire would be too busy defending itself than getting involved in foreign adventures.
Romans, maybe. If Alexandros' empire fragments earlier - which it ought to - then one of the Successors will probably go off on a fun conquering spree and may seize Rome as his power base. Kind of like Pyrrhos but without the other stuff to deal with, and hopefully without the ADHD as well. Since an integral part of any Hellenistic empire would be control of, well, Hellenes, the Alexandrine Empire will probably want to seize Megala Hellas, which brings them into conflict with Rome, a Rome that had major issues with a renegade Hellenic genial military madman with a misthophoroi army. As for Qarthadast, well, I've already said that they were incredibly weak during a large chunk of this time period. They had major trouble coping with Syrakousai, so the Alexandrine Empire should be able to trash them. I can see a Keltiberian/Qarthadastim successor state in southern Iberia arising if the Hellenes take the Qarthadastim metropoly, and of course whatever successor state to the Alexandrine Empire that arises in North Africa will be highly Semitic-influenced.
Nanocyborgasm said:
the combined pressures from the East and West would likely cause a collapse of the Macedonian Empire around 100 BC.
That's an awfully long lifespan. It should last for far less time than that.
Seems like you said a lot of stuff that was already in this here thread, except for the expeditions business, about which I have serious doubts. The nice thing about Alexandros' 'later years' is that he doesn't have any external threats of any significance, and thus can easily afford to go on another conquest trip. The viability of controlling such territory over a long period of time is low, but when the Alexandrine Empire breaks up, places like Qarthadast (and Rome, if it is conquered) will probably become Successor Kingdoms a la Egypt of the Ptolemaioi, Seleukid Syria, and Antigonid Makedonia in OTL. Essentially, Alexandros' main impact, in keeping his eponymous empire together, would be to prevent the Hellenes from infighting for
just a little while longer and thus improve Hellenistic cultural power throughout the Middle East and Mediterranean via conquest. But nobody will disagree about a breakup.