Democracy never had a real chance in ancient times
Agricola, there is a call for you. 8 generations of Athenian citizens want to discuss with you about 2 centuries of a certain political system with you and... seriously, they look quite angry.
With new tribes all over Italy, and winning socii representing these new tribes, you have to vote all over Italy, decentrally.
Well, is there any precedent for this? The Italics had their consuls during the Social War in OTL - but how were these consuls elected? Certainly not decentrally as you suggest it.
Either the consuls were
1) elected by a central people's assembly in Corfinium (copy of the Roman Comitia) or
2) elected by the Italic Senate.
The Italic Republic was a confederation of cities, and if you take a look on Greek confederation, there was a council consisting of delegates from the different cities, and magistrates administrating the confederation. Sometimes these magistrates were elected by a central people's assembly, sometimes by the council.
But I never heard of decentral elections in ancient times.
Remember: Our parliament evolved out of feudal assemblies representing the different orders of medieval society, and not out of people's assemblies. A Roman parliament will have to develop out of the Senate.
Where do you believe these Samnites or Ertruscans are willing to elect their tribunes?
To be fair, there is ONE example of representative aristocracy (not democracy) in ancient times. Augustus is said to have tried decentral elections: He made it possible for Italian decurions (members of city councils) to cast their ballot through postal voting, so that the Italic upper class could take part in politics without coming to Rome (not that it would have changed something to Augustus' dictatorship, but it was certainly conceived to be a legitimacy boost for the Principate).
Obviously, this system failed (since we have no evidence it was actually used), probably because absantee voting was just to complicated in ancient times.
@Agricola Are you aware of all the difficulties of decentral voting? The Roman assemblies gathered often - for the election of 6-7 magistrates a year + the elections of religious officials + certainly at least 10 laws a year.
So roughly 20 times a year, elections have to be organized in each city. Controlling the elections was the task of the consuls - but Italy had hundered of towns! Also, remember that the membership in a tribe was hereditary - so each city has to control if the citizen has the right to vote there. Than the votes have to be counted and the repport sent to Rome.
Also, who should the decentral assemblies vote for? There are no real newspapers in Rome - the people in the cities maybe know the local politicians, but they need to know at least 30 different Roman candidates changing at each election. And the candidates have only a limited amount of time, so they'll concentrate on the most important city (still Rome), without becoming known in Italy.
Such a representative system is simply utterly impossible before the era of the politicization of the masses through newspapers and better communication.