Polybios praised the roman constitution, because it was a "mixed constitution" with monarchic, aristocratic and democratic elements:
- monarchic: the undivided power of the roman magistrates (undivided in a modern sense: legislative, executive, iurisdiction)
- aristocratic: the senate as Advisory Councils of the Elders representing the aristocratic families
- democratic: the comitia as legislative
Even if this is partially true for the almost uncodified de iure constitution, it is fully wrong for the even more uncodified de facto constitution of the roman republic. In best case during the times when the roman constitution worked perfectly, the roman senate was the one and only prime executive, legislative and iurisdiction with fully undivided power. The magistrates (even the consuls) were just servants of the senate acting closely micromanaged on behalf of the senate, and the comita decided, what the senate told them to decide. When this succes model stopped working, the republic failed.
However, the monarchic element was absurdly strong with the promagistrates, because
- they had no colleague able to veto
- they had no tribunes able to veto
- the quaestor, even if officially an independent magistrate with an independent mandate of the senate was usually just a clerk of the proconsul, because there was no senate nearby. So he could not resist against the higher auctoritas and dignitas of the proconsul.
- the proconsul was consul (military), praetor (iurisdiction and legislative to a certain extent) and aedil (infrastructure) in one person. Plus he usually controlled the quaestor (finances).
- beside the non-exisiting collegiality, also the annuity was often violated by extraordinary commands (2-10 years).
This have been perfect conditions for corruption, exploits, embezzlements and in worst case usurpation.
The major tasks of a proconsul have been:
- external security (border provinces only)
- internal security
- iurisdiction
- supervising the self-administration of the cities
- collecting /supervising (not gathering) the taxes
- infrastructure projects
Of course you could split that like in Rome amongst multiple magistrates: proconsul (military), propraetor (iurisdiction and supervising), proaedil (infrastructure) and proquaestor (finances). For non-border provinces, with just a small army (usually just a few cohorts) you even do not need a proconsul, because inner security was more the responsibilty of a praetor anyways.
But there are some issues coming along with such an approach:
1. Splitting the military command in a province between 2 proconsuls might be detrimental. The romans made bad experiences with 2 consuls leading one army (e.g. Cannae).
Multiple praetors (even more than 2) makes a lot of sense. Provinces had up to 7 conventi (places or cities of trial). More praetors would reduce travelling significantly for all. Multiple aediles make perhaps not that much sense, because infrastructure outside of the cities responsibilities was not that of a big deal. Actually the romans never implemented a proaedil in the provinces. But multiple questors might make sense for better control.
2. The introduction of all 4 magistrates does not solve the issue, that the proconsul or propraetor with the highest auctoritas and dignitas is able to dominate the others, without a senate nearby supporting lower magistrates and controlling the highest.
3. the romans had not enough aristocratic personel. That was one reason, why they invented the promagistrate (just a pro-longed magsistrate). Roman magistrates were officially honorary offices. Even 2 consuls in Rome in peace times shared the job on a monthly basis in order to get enough time for their private business. So roman aristocrats were not willing to dedicate their life to the state like a prussian clerk.
This means, such a model is just possible, if you use equites at least for the lower magistrates, too. But this would be a big change for the roman aristocracy. Because this would have strong effects on the roman society, the senate and the aristocracy and their clients in total. Some roman arsitocrats saw the need for more lower magistrates, e.g. Cicero in his rather naive recommendations for a reformed republic.
The principes actually did it. They decoupled the finances from the governors by implementing independent equestrian financial procurators. In the senatorial procinces these procurators have been just responsible for the patrimonium caesaris. However these procurators often peached the governor and were an instrument of control. But corruption still happened. Especially if governor and procurator worked together in criminal harmony.
So the idea is good. But the implementation needs a bit more thinking. And more consideration of the roman mindset. Why the hell, the roman aristocrats should get this idea or accept and follow this idea? This is always the mother of all questions with every good idea on this forum. Nevertheless, I am convinced that more division of power or balance of power in the provinces - even if never in a modern sense - is badly needed for a more stable rebublic. Even for a more stable principate.