So this post is more of a what-if than my earlier ones; I've been humbled by my apparent lack of knowledge in certain subjects!
What If Hephaestion survives his sickness? Let's assume that he really had typhus, as some postulate, and that overeating (meaning just a small-average meal for a healthy person) upon near-recovery is what did him in; his doctor simply pays attention and doesn't let him eat so much, so he survives.
What If, then, Alexander the Great invites his wife Stateira (daughter of Darius), he sister Drypetis (wife of Hephaestion, daughter of Darius), and their mother Sisygambis to Babylon as a matter of course, before his planned invasion of Arabia.
Then, Alexander dies as in OTL.
What changes do you see, with Stateira carefully watched and possibly warned of Roxane's viciousness, and Hephaestion acknowledged by all as Regent (for Stateira's child, and Roxane's, and hell, depending on Meleager's actions, Philip Arridaios), his closeness to Alexander being well known.
This assumes that Stateira was carrying a male child, and that it was Roxane who poisoned her and not just some sickness. I think it can be taken for granted that Hephaistion will be universally acknowledged; Perdiccas wasn't too popular, and Alexander wasn't too close to him, and yet Peithon, Peukestes, Seleucus, Arybbas, and Eumenes (all powerful men) followed him into disaster on the Nile.
Also assume that Heracles of Macedon was an illegitimate pretender, with no ties to Alexander.