Hephaestion would have been made Alexander's regent if everything goes the same as OTL, essentially taking the role Perdiccas did IOTL. His closeness with Alexander made a lot of the other commanders jealous of him, so he had a lot of enemies within the Macedonian military aristocracy, namely the popular general Craterus. Eumenes - who was one of Perdiccas' best lieutenants and closest advisers, and grew to be one of the great figures of the early Diadochi Wars - and Hephaestion had a bitter rivalry. On the other hand, in the eyes of some, he would've had a clearer mandate to rule as Alexander's successor than Perdiccas because they were so close. I imagine Hephaestion surviving, however, is ultimately more likely to be a bad thing for the preservation of a united Macedonian empire than him dying like IOTL. It's hard to say that given how OTL went, but I feel like Hephaestion had too many enemies and too few friends to rule effectively in the name of Alexander IV and Philip Arrhidaeus, and that because of this things will disintegrate quickly.
It'd be interesting to see how Hephaestion handles the Partition of Babylon - if he tries to deny Ptolemy Egypt, and how the rest of the empire is divided compared to OTL. How that plays out affects how the conflicts thereafter play out.