So let's talk a bit about your model of governement.
Its not my model. Its the work of the great emperors Trajan and Hadrian
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I understand, that you introduced 2 co-emperors or super-legates with the praefectus orientis and praefectus occidentis. I deem it advisable to install a 3rd praefectus illyricum. Imagine, while Hadrian fights in Cyrene, the Germans cross the Danube. Responsible commanders with decent armies are needed to avoid one of the biggest reasons for usurpations in the 3rd century: The legions themselves supported by the provincial elite acclaimed (and enforced) a local legate emperor, in order to get optimal conditions to defend their homeland.
Like almost every important reform in history, the prefectures were not the invention of one man. Trajan created one in the east, Hadrian one in the west to protect Britannia and the Rhine. A third prefecture on the Danube will be formed when the necessity emerges. But, as you said, it has the goal and the effect to centralize and to simplify the defense of menaced regions of the empire.
As distinct from the late empire multi-emperor model, not the entire empire is divided into praefecturae. Because your praefecturae are not a new and consistent level of civil administration, but primarily of regional military defense. Pacified regions like Spain, Macedonia or Asia must not belong to a praefectus. Also minor fronts like Africa or Aegyptus (1) can remain the responibility of a classic Legatus Augusti pro praetore. Furthermore all new civil magistrates in the praefecturae should report to the emperor. This helps to reduce the power of the new praefects to the absolute minimum they need to defend their front. In the late empire we got 4 emperors with 4 civil praefecti and at least 4 magistri militum. In your TL we have just a few praefects reporting to one Augustus. This should reduce complexity greatly and increase manageability.
Yes, simplicity and a logical structure are the aspired values of the new military organization. But as separtion of military and civil power (magister militium - comes - dux/praefectus - vicarius - praeses) wasn't known in the 2nd century, the point that all civil magistrates report to the emperor from the beginning is quite unrealistic. The first target of the emperors will be to maintain the centralization of the military command, while local authorities can slowly emancipate through the different provincial institutions created to check the prefect's power. Do not expect all at once. It's though quite possible that some of the imperial provinces outside of the prefectures will be returned to the "senate and the people" as they are considered as safe (the imperial provinces of Hispania and Asia), while the last senatorial provinces with military presence become imperial (Numidia).
You also introduced more local division of power. It makes a lot of sense, that these super-legates have no power of civil iurisdiction and adminstration and no access to unlimited funds. I also like the idea of a consilium prafecturae. So far my idea was just to strengthen the already existing consilii provinciae. But to introduce a consilium on a higher level makes sense, too. Every roman magistrate had a consilium. The emperor had one, and the duumvir of Pompeji, or the procurator Alpes Maritimae had one, too. But these were just informal advisory boards, hand-picked by the magistrate himself. To appoint these consilii offcially and gave them actual power, makes again a lot of sense.
Divide et impera. The different consilia (inspired by the conseil préfectoral and conseil général of Napoleon) are not born out of an ideal of letting the local elites take part in the prefecture's affairs, but the outcome of the consequent thought that competition and mutual suspicion are the best way to stop a possible united opposition to the central power in Rome.
I understand that Gaius Quietus is a Caesar and heir, but Quietus Commodus is not. How lucky you, that Commodus was old and experienced enough to do this job at all. Is it really wise to appoint that young men? Furthermore an important question is, if a praefectus has to be Caesar or not. Is it preferable to have 3 Caesars (assuming you introduce the 3rd praefectura)? Is it preferable to have just 1 Caesar? And is it advisable that none of the praefects is Caesar, because the emperor has no son or the son is too young. How could a working model of succession look like? How to avoid, that an emperor dies without an widely accepted heir. How to avoid, that multiple Caesars, prefects or sons of them compete about succession in a civil war like in late empire? And how to avoid, that an incompetent son of the emperor like this other Commodus becomes heir.
I thought of a clear succession order: when acceding to his office, the Augustus of the emperor, ruling from Rome or, more generally, from the western part of the empire, chooses one trustworthy friend he whish to be his heir as
Caesar and
Praefectus orientis. As a heir, he owes his whole position to the ruling emperor and knows that he will be emperor at some point; this reduces the risk of an usurpation. A third person, chosen by the Augustus will be adopted by the Caesar and appointed
Praefectus occidentis - he is very young and inexperienced, and is hence educated in this direction during his youth. The current Augustus is Hadrian, Gaius Quietus is Caesar and Praefectus orientis, Commodus is Praefectus occidentis.
On the issue of Commudus and Quietus: Quietus gained some experience in the west since Trajan appointed him governor of the west.
To ensure the fidelity of such an important commander, Trajan thirdly stipulated that Hadrian would adopt the son of Quietus, who was in this way made heir of Hadrian (and of Trajan). The presumed heir should act as "viceroy" in Gaul and Britanny to gain some experience before becoming emperor himself. In later times, this special sequence would evolve in an exactly defined order of succession to the imperial throne.
So, Gaius Quietus has almost ten years of administrative and military knowledge when becoming commander in the east (And he isn't that young: Lusius Quietus' father was already soldier in 40 AD, so Lusius Quietus was maybe 70 years old in 130 AD, and his son is now 40 years old; Commodus is only 30 years old). The choice of Commodus might be disadvantageous, but this is how history works: men make mistakes.
So still a lot of issues to solve, but I guess you are on the right way.
Time will tell.
Perhaps you should think about a more powerful consilium principis. Could be formed by the most powerful magistrates after 3 years of service. Perhaps as 1 station of a rotation model. Also think about a central field army (exercitus praesentalis). An emperor without his own force is a lame duck. And a central mobile reaction force (mobile means fleet support) was one of the good ideas of the late empire. But how to avoid, that this central commander of all powerful people usurps? Hint: does a central field army really need a single commander?
Hm. The emperor has already the two central fleets, the Praetorian guards, the Cohortes urbanae, the Vigiles, and, most important, the Danubian and Illyrican legions in his proximity and under his direct command. That's also why I'm opposed to a third prefecture on the Danube. The emperor needs some troops controlled by himself to put down putschists. Also, the creation of a central exercitus praesentalis requires the weakening of the units at the limites imperii. And why should the Roman principat form a comitatenses - limitanei army if the barbarians beyond the borders cause no dammage - at least until now. Though, I could image a gradual reinforcement of the Praetorian guard (like the Imperial guard of Napoleon: from 4 178 in 1800 to 112 482 in 1814), which will lastly be somewhat like a "comitatus" of the emperor.
And please state our ideas on the consilium more precisely.
