It could be more plausible that Octavian doesn't believe that anyone else on their own would be able to be responsible for the whole Empire, and re-engineers the Republic. In this case his motivation is the survival of the world he has forged.
The issue is "How"? It may be that when he turns 65 (Jesus this guy lived forever), that he orders substantial republican reform, creating a form of "Federal Republic" - A Senate in all the "Republican Provinces" - Essentially every province that he doesn't judge direct control over as essential to the Empire - as well as establishing a non-Imperial military structure that does not encourage rebellion, but does reward expansion (the details of which I am still fuzzy on). This would probably leave Aegyptus, the Bosphorus, Sicily, the straits of Gibraltar, and probably Italia proper under central control.
Then the Princeps of the Republic (or Grand Republic?) is the guy in charge - each Senate determines their own Princeps (Princeps of the Republic in Gaul), and whilst those Princeps have power, the Princeps of the Republic can only be chosen from among those members, perhaps by representatives of each Senate.
The problem still remains that Octavian would die the wealthiest man in the republic. He would have to give away vast amounts of wealth prior to his death, and on his deathbed, to prevent his successor from becoming instant Princeps of the Republic. Perhaps a huge land giveaway to his soldiers?
The division of powers, based on the premise that he doesn't believe anyone else has the capacity to rule the full Empire, probably means that there is a lot of decentralisation to the provinces - with the Military and Territories of the Republic under central government, and the Military problem still needs to be resolved - is the Princeps of the Republic akin to the Commander in Chief, more authoritative than military man?
Security may have to fall under central control to prevent those Republics safe from danger from neglecting it, or refusing to contribute. (Italia comes to mind). But he'd still need to develop a nimble military system.
It isn't a small task, but short of a Federal Republic, with a balance of Territories and Provinces, I'm not sure what I can contribute to the idea, beyond glee at more democracy.