I remember there was an early Alt Hist article by the historian Arnold Toynbee on what could have happened if Philip had not been assassinated in summer 336 but had discovered or suspected that Alexander was involved in the plot and executed him, then had gone on to invade Asia Minor himself as planned. This more or less has the invasion developing as per Alexander's plan as far as the offer of half of the Persian Empire in return for peace by Darius in 331, by which time Macedonia has conquered Asia Minor, Syria, Palestine, Phoenicia (essential to cut the Persian fleet off from its main harbours) and Egypt. In OTL when this offer was made Parmenion, Philip's ex-lieutenant who was around P's age, advised Alexander to accept it - 'I would if I was you'. Alexander replied 'So would I, if I was Parmenion', rejected it, and went on to fight and win the battle of Gaugamela near modern Mosul and then take Babylon and Persia. In Toynbee's version, the offer is made to Philip and he accepts it - and the frontier between the Greek world and the Persian world is fixed at the Euphrates, more or less at the Roman/ Parthian frontier of the C1st AD. So we have a Macedonian-run and colonised Greek-led , Greco-Asia Minor/ Aramaic/ Egyptian kingdom from Macedon to the Euphrates, as a sort of proto-'Byzantine' state, acting as long-term hegemon of Greece; and Philip lives to his early 80s like Antigonus 'Monopthalmus' and Seleucus and dies c.300, succeeded by his son by his last wife, Attalus' daughter, who he had married in 337. (In real life Olympias had her and her child murdered once Alexander was on the throne - this 'Borgia-style' family saga would be great on TV, possibly adapting Mary Renault's novel 'Fire From Heaven'. The modern film version of the Alexander story with Angelina Jolie as Olympias was not much good in my view.)
This is more or less my guess as to how a Philip-run invasion and campaign would have worked out, as Philip was not a man to run risks unless , for one particular strategic feint to confuse an enemy, he already knew the landscape intimately (eg in his wars in hilly Phocis around Delphi with the Phocian league and later with Athens/ Thebes in 339-8). Strategically he needed to take Phoenicia with its ports to cut off the enemy fleet and deny it its bases, and also to keep the local fleet in his hands after the war so it could not be used by Persia to restart the war and attack Greece by sea - I think Alexander was already running dangerous risks as early as 333 by not bothering to respond to the Persian navy at sea whatever it did in the Aegean (and in risking his life at the Granicus), long ahead of his dubious strategy of marching his entire army across the Gedrosian desert from the Indus back to Persia in 325 without a Plan B in case he got cut off from Nearchus' fleet and their supplies. Possibly if Philip thought he could not trust Darius to keep to any terms , or had proof that D was behind the 336 assassination-plot, he would have sought out a final battle in Mesopotamia to smash his army and make sure that Persia would collapse - or even taken Babylon and gone on to sack Persepolis and break up Persia. (The 'Trajan strategy' of 116-17, four centuries early?). But I can't see him risking a full occupation of Persia; a Greek state running Mesopotamia and the entire Levant as far as the Bosphorus, a sort of proto-Seleucid realm (minus its Persian outliers) but with Macedon too, was feasible. Possibly Alexander would have done better to stop in 330 after overrunning Media and the Caspian shore, with Darius dead, or just to have tackled Bessus in Bactria in a swift 'dash' into the region in 329 then pulled out and left the Hindu Kush alone and never gone to India. The list of corrupt or incompetent governors who he left behind and had to get rid of in 325-4 (plus Harpalus looting the treasury)is an indictment of his governance that his fans tend to forget about; was Philip a better civil administrator if not general?
A revival of the imagined Philippan strategy for the Levant by Rome, eg by Julian in the 360s , is also an intriguing prospect: breaking up Persia but not ruling it directly.