A proper legislature would certainly take quite a while to evolve from the scenario I propose. As a rough outline: ...
Your concept comes pretty close to my thoughts. I am just starting with the
Optimates in the senate, who are trying to stabilize the roman constitution, via strengthening the senate, like Sulla tried and like Cicero outlined in
de legibus and
de re publica up to a certain extent.
One major basic problem of the roman republic (amongst many others), was the huge discrepancy between the de iure constitution and the de facto constitution:
De iure (according to a few constitutional laws and the mos maiorum):
1. The magistrates are the executive of the republic. They govern the state and promote bills.
2. The senate is an advisory board. The senate advises the magistrates on strategic decisions and bills.
3. The comitia elect the magistrates and decide on laws
De facto ( during the time this aristocratic republic did really work):
1. the magistrates are agents (henchman) of the senate. They are not allowed to decide about anything beyond usual daily business. They are also not allowed to promote a bill to the comitia, without a preliminary decision of the senate. If a magistrate acts against the will of the aristocracy represented by the senate, his colleague or some of the (aristocratic) tribunes will block him easily for a year and afterwards his career and social standing is ruined.
2. The senate is the highest executive, legislative and in some cases even iurisdiction. Magistrates are controlled closely. If needed, a senatorial committee (decemviri or any other number) supports the magistrate. The decision of this official consilium of a magistrate is binding. If there is no clear responsibility of a magistrate for a specific issue, such decemviri could act independently as an executive body.
3. The comitia should just vote about bills which already passed the senate and are supported by the majority of the senate. Due to the fact, that the upper classes have the majority in the comitia centuriata, the final decision should be obvious. The aristocracy also ensures, that just proper candidates are nominated for election. If this does not work occasionally, the censors could ensure, that an undesired ex-quaestor becomes no member of the senate. The comita plebis is controlled by many aristocratic tribunes, which still have hope to make a career in the higher magistrate.
This huge dircrepancy of law and political reality and the scrapiness and low grade of codification of the roman constitution, opened possibilities to bypass the constitution and destabilize the republic. It worked while the aristocracy was fairly united about the big political questions or at least able to reach a consensus mediated by the
consulares of the powerful families. When the aristocracy became unbalanced in terms of wealth and power due to the expansion, single
potentates (but also bankrupts) started to use the obvious flaws of the roman constitution for enriching and empowering themselves.
Now let us assume, that the
optimates in the senate, try to reform the republic in a way, that the de facto constitution becomes the de iure constitution and is not longer bypassable. What could be the proposal?
1. they strip the comitia of its legislative power, like Tiberius did. All decisons are now done by the senate. Every bill is promoted to the senate. Also the magistrates are now elected by the senate. Senators are still nominated by co-optation, like it theoretically always has been. This was also the usual process in most other roman cities.
2. Of course this would bring a huge and loud populist opposition to the scene, which is not willing to accept this approach. Now your concept comes into play. As a compensation and due to the strong roman tradition regarding "democracy" (mos maiorum), the role of the tribunes is strengthend and the comitia plebis survives as the only comitia.
3. But this new role and reponsibility of the tribunes is regulated heavily:
- Tribunes still have their right of veto. But just if the majority of the tribunes agree, which was not that unusual as I explained above.
- Tribunes should also participate in the election of magistrates by the senate, due to the disempowerement of the comitia.
- The tribunes (now called Comitia Tribunorum) still participate in the sessions of the senate, like they always did. They also have the right to discuss, not just announce a veto. This integration of 2 chambers in one session is very different to a modern 2 chamber system.
- Tribunes are still elected by the comitia plebis, but the comitia looses its right to decide about bills. One reason (like for all comitia) is, that over 90% of the roman people are living outside of Rome mainly in Italy but also in the provinces and cannot participate in frequent votes.
- Due to this geographical distribution of roman citizens, even for elections the romans use a system Augustus used just once. I mention Augustus here as a proof, that this approach is not fully against the roman mindset. All roman cities get a list of candidates (for tribune) and vote onsite. The results are forwarded to Rome via local magistrates as couriers. Sounds pretty similar to the american system, but just similar.
- In order to reduce the effort for such elections, tribunes are elected for 3 years (the average time of service of the later imperial governors or procurators sounds appropriate). They can be re-elected, like it sometimes happened in the early republic. But they are not allowed to become senators or magistrates. This leads to a situation were mainly the roman equites apply for tribune.
- During this process of implementing this new "Comitia Tribunorum" the number of tribunes is heavily increased. Also to balance a bit with the 300+ senators. Well, 300 tribunes would be a bit too much. But 10 are not enough obviously to replace the comitia.
- The tribunes also have still their right to promote bills. But due to the disempowered comitia they forward their bill to the senate, which is now the legislative power. Every bill of the tribunes can be blocked by the majority of the senate (veto) like every bill of the senate can be blocked by the new comitia tribunorum (veto).
- I am not sure, how to deal with the "ius auxilium". That was a tribunes right to step in and block every action of a magistrate against a roman, if the tribune is convinced, that this action is against roman law. This was the initial right of the tribunes, the most important one, and their daily business. However, we do not read much in ancient sources about this daily business, because ancient authors focus on the big political cases.
Voila, here is a 2-chamber system, which is not fully beyond the roman mindset and could happen in a well thought longterm alternative history.
This model includes also some first steps into the direction of representation for the tribunes. Well, the tribunes always have been representatives of the people. Just not in a modern sense. In a next step you could introduce an even more representative system, where the fully romanized roman provinces (e.g. Narbonensis, Baetica, Sicilia, Africa proconsularis,...) with a high amount of roman citizens directly elect their tribunes and send them to Rome, because the process of sending couriers to Rome is too unpractical looking to the long distances. This could now lead to a next step where also the italians are asking for directly elected tribunes per region. And finally we got a kind of roman House of Commons with representatives of the people.
The model also does not touch the senate as a 1st chamber of the aristocracy build by co-optation. The senate still has to deal with these annoying tribunes. But now in a much more regulated manner, which should lead to a more proper legislature.
Nevertheless, this is just one of many reforms needed to stabilize the roman republic. Here just stabilizing the legislative. We still have no division of powers and the senate is still the highest executive body of the republic. And the much more dangerous and challenging problem comes with provincial administration and military command. I also used some concepts, which were feasible during the early principate and are questionable in an earlier timeframe.