Why do you think, the implementation of big praefecturae would be beneficial? Do you see advantages in military defensability or in civil administration?
And if you talk about dioceses. Do you mean an additional lean supervisory body like in late empire or do you want to replace the provinces by bigger dioceses; and why?
I guess Augustus could have done a lot, if he would have seen a good reason to do so. Of course he has to keep the illusion of a republic up. For example, if praefecturae are just for a better civil administration he could have implement correctores with a rather temporary character. And later emperors would then develop this system to permanent praefecturae. If you like to implement something like a Legatus Augusti per Oriens commanding all legions in Syria and Cappadocia and other asian provinces, this could have done anytime based on his imperium proconsulare. But you have to answer the question, how to avoid usurpation, if you have 3 big military leaders at Rhine, Danube and Euphrat.
Looking to dioceses, Augustus actually started to implement something like that, with the procuratores (finance). The territory of these procuratores was usually much bigger than a province, e.g. Gallia without Narbonensis, plus Belgica and Germania. This was surely beyond any republican tradition, but it was done, because Augustus saw a need to do it.
If you like to change the structure of the roman empire (military, civil or social), Augustus is the perfect time to start. But afterwards you have to wait 100-200 years, until this new structure fully develops.