For one, his reign would lack the legitimacy of longstanding Arab monarchs since his family doesn't have a long history of being royals.What if when he became succeeded his father instead of Al-Assad becoming president he decides to become King of Syria and grants a constitution of Syria like Morocco's or Jordan's where the president retains quite some power?
Or as Emperor Bokassa I, for that matter.Making himself king would make al-Assad almost as megalomanic as Idi Amin.
He needs a good dose of Harry Potter magic for that, though; after all, Israel has nukes and he doesn't.The only way he would any legitimacy is if he pulls some napoleon shit and defeats Israel in a major war and unites the Levant