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.
I do not expect that everything happens at once. It is quite hard to change the roman mindset. I fully agree, that an emancipation of local authorities helps to control these powerful central magistrates. But be careful. There is a point, where this must lead to separation, the worst case of usurpation! Centralism is one of the key success factors of the roman empire. Too much federalism leads to chaos.
Regarding the division of civil and military power. It actually started with some measures like Nervas introduction of the
advocati fisci and other measures in the 2nd century. Diocletian was not the great inventor. He was just very good in picking up older ideas, integrating them, and implementing them on a broad scale. But after all he did harm the empire more than he did good (militarization of the society and his genious but longterm detrimental taxation system).
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.
So far so good. In theory. But what happens, if afterwards a son of the emperor is born? Who decides, that he plays no role in the succession? You know the story about the most succesful usurper of the roman empire: Constantine I! How to avoid such an event by constitution or better by actual balance of power?
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.
A central field army is not just good to protect the emperor. This was perhaps the main reason for Constantine. He almost ruined the empire militarily with his paranoia. Strategically, the central army is the force which re-enforces the praefectus orientalis, ocidentalis or illyricum, if the enemy is too strong. The alternative is weakening one front, in order to send troops elsewhere. And you know, how this ends.
Furthermore, it is time to wake up. It is 124 AD and the Danube border is still under a heavily decentralized military command. Ok, the romans don't know whats coming. But after the desaster, they should know, that they have to reform the military organisation at the Danube in your TL. Like they did in the West and East.
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
Not necessarily. The economy of the roman empire could be strong enough in order to finance another 50-60000 soldiers. Just analyze the roman economy and avoid the mistakes done by several emperors since the early principate.
And I did not talk about comitatenses and limitanei. There is no need to introduce this split. Actually it already existed since the early principate: the auxilia guarded the border and the legions were hold back as a reserve. Just the distribution of these smaller "regional field armies" was different during the early principate. I guess strengthening the praetorian guard and adding some legions with a fleet to move them fast like Severus did, should work in a first step. Brundisium comes to mind, or Mediolanum.
Finally the split into limitanei and comitatenses was not a bad thing. It strengthened the roman army! But I agree, that it has to happen step by step. If you choose to let it happen.
And please state our ideas on the consilium more precisely.
You mean I should describe in detail, how the consilii provinciae and the province administration could be changed for more federalism without violating the ancient mindset, and without ending in separatism? And how a consilium principis controlling the princeps could look like?