I guess we have to structure the answer to this question a bit.
A. Circumstances of the soldiers
The republic never established a clear law for the supply of the soldiers, which includes
- regular payment by the state
- mandatory pension scheme
- fair and predictable share of loot
Augustus fixed that. Even if I did not found distinct sources how the principate dealed with loot. However there were rules about it.
The principes still had to deal with dissatisfied soldiers mainly by 2 reasons:
- sometimes unstable food supply
- too long and/or prolonged service times disregarding existing rules
But these seems to be solvable problems.
Key is, to bind the soldiers to the state, not their general. Augustus did that with measures covering the points above. Furthermore he controlled the promotion of officers from centurio onwards. He also established an equestrian career to balance the power of the senatorial legates. A process, which was finished by Claudius and the Flavians finally. And he introduced the emperor-cult for the soldiers.
The latter was a terrible mistake. The emperor became the new superman. He and only he could solve all problems. Actually this was the truth. The emperors proved more than once, that they can solve every military, administrative or juridical issue in the provinces. He must have been a god! This led to a desaster in the 3rd century. People saw no other chance against the multiple invasions than to ask for the emperor himself. Due to the fact that there is just one emperor, they were forced to appoint a local one. Usurpation often was no greed but a necessity.
After all, the roman legionairs knew very well, that they are able to entrhone an emperor since the civil war from Julius Caesar to Augustus. And they never forgot it. The roman army changed from the republican army of citizens to a professional army since Augustus and then to an independent military society. There are a lot of reasons, why these changes happened and were even sometimes necessary. And it is hard to say, how to avoid the disadvantages coming with these changes. Just going back to the old republican recruitment system is not feasible for an empire. And barbarization is a myth before the desastrous battles of Adrianople and Frigidus. So this leads to the first question:
1. Which longterm changes of the roman military were disadvantageous, which measures caused them and how to avoid them.?
B. Succession
As already mentioned, in the first 2 1/2 centuries from Augustus to Severus Alexander usurpations were rather seldom. The soldiers even declined to usurp against a legal emperor. Usurpations usually occured, if an emperor died with no clear, obvious and accepted heir. Another reason was, that an emperor became unacceptable. Both is inherent in the system of hereditary monarchy and therefore unavoidable if a monarchy. Dynasties will die out or produce idiots on the throne. No doubt about that.
Egon Flaig explaines the fundamental reason for roman usurpations in his habilitation "Den Kaiser herausfordern" (To challenge the Emperor). The roman princeps was never legitimized. He was just accepted by 3 acceptance groups: military, senate and the people fo Rome. And it was a matter of roman culture and civic duty to withdraw this acceptance and challenge the emperor if necessary.
The designated heir of the emperor had usually the best chances to become the next princeps. However, there was no clear rule for succession. Not for appointing a princeps and not for disbanding him if neccessary. And there was not one instance/authority , which was:
- widely accepted
- powerful enough to ensure its decision
The roman senate had some power, but this was behind the scenes, often in coniunction with the fact, that all generals were senators too. Also the power of the roman people is often underestimated. Their main requirements to an emperor in order of importance was respect, supply of the city of Rome and dignified behaviour. The roman people were just once directly responsible for an usurpation, but bad mood in Rome led to braver senators and growing unrest in the legions. So senate and people had power, but more indirectly.
Only the military was powerful enough to enthrone and dethrone an emperor. Unfortunately there was not one central accepted and powerful military instance to ensure a smooth succession. The praetorians had a big influence during the julio-claudian dynasty but finally they (must have) failed. Usually the 3 big armies at Rhine, Danube and Euphrat competed about the right to enthrone a new princeps.
This leads to 2 more questions:
2a. Is it possible and preferable to establish one central military power, which rules the state?
This must not be a big central field army necessarily. I would also not call it a military dictature, because roman officers never have been only militarians. They were generals, governors and judges in one person. Also their tribunes and centurions and even a lot of principales and miles gregalis had military and civil duties. There was no clear distinction in roman government between soldier and clerk or executive, military and jurisdiction until late empire.
2b. Is it possible and preferable to empower the senate and/or the people of Rome to fullfill this role of an accepted council and how to make it powerful enough?
Please don't tell me, we need to fix that in the constitution. Thats modern thinking. History tells us pretty precisely, that the rule in Rome always was: Constitution is, what is enforceable. I am convinced, that there can't be stability in Rome without a stable power, regardless of the constitution.
On a sidenote: due to the fact, that generals are senators and vice versa, the 2 questions / options do not make a big difference, if you think like a roman did.
3. Pressure
The fall of the principate has less to do with politics than with changes and structures. Finally the system of the principate, which was relatively stable for over 200 years failed dramatically, when it came under pressure mid of the 3rd century. It collapsed like a house of cards.
There was external pressure due to more powerful (population, wealth, organization, technology) barbarian tribes, the change of the reign in the East with the more aggressive Sassanids and therefore a war on multiple fronts. Actually up to 6 fronts at once (Rhine, Upper Danube, Lower Danube, Euphrat, Africa, Britain, Egypt & Palaestina)
There was also a lot of pressure due to changes (sometimes decline) in finance, economy and society. There are a lot of longterm processes which turned the former strong roman powerhouse into a mess.
The question here is:
3. How could the romans avoid partially or lessen the impact of these changes and is there a political system which would not collapse that easy under pressure?
The following system of multiple emperors and finally a divided empire had not less usurpations but different ones like usurp against the Caesar not the Augustus, or usurpation of a magister militum by establishing a puppet-emperor. Additionally the late roman emperors tinkered with central field armies for their security, divinity, enthroning ceremonies, more centralism and a lot of other stuff. But everything failed. The late roman emperors also failed to manage the economical and social issues. Some of their measures were genious, others made things even worse.
So, in order to answer the question of the OP, I would like to recommend, that we focus on the 3 questions I underlined above.
