I subscribe to the theory that Heracles of Macedon was Alexander's son.
Given that Roxana's child was not yet born at the time, and Heracles was a youth (18 IIRC), and we assume he is acknowledged as Alexander's...
What would happen?
What you need is someone who can't claim the entire thing for himself, but can act as a regent of sorts and is loyal to the Argead line. Eumenes of Cardia, the Greek among Macedonians l, would fit this role well.
Eumenes would never be accepted as Regent of the entire Empire. Look at how much trouble he had keeping his soldiers loyal. No the best thing to do would be to have Hephaistion survive his illness. That is one person we know would be loyal to Alexander and his bloodline unequivocally.