Elections in the Modern Roman Empire

[Verily]

Is the Empire pagan or Christian? That would really affect the political atmosphere...

[Verily]

National Imperial Party: weakly Latin nationalist, populist, perpetually largest party, strongest in Italia, Gallia, and Britannia as well as Dacia and Taurica
Latin Left: Latin-speaking but non-nationalist, left-wing, anti-monarchy, strongest in Hispania and North Africa as well as the Scandinavian and Arabian hinterlands
Greek Socialist Movement: weakly Greek nationalist, left-wing, strongest in Anatolia
Free Republican Party: cross-lingual, free-marketeers with strong social-liberal bent, strongest in Hellas and the major urban centers of Syria-Palaestina and Italia as well as Aegyptus
Party of Greeks: Greek nationalist, support diffuse in all Greek-speaking areas
Party of Faith and Traditions: cross-lingual, supporters of religious faith in all forms, strong Platonic influences and therefore actually socially moderate to liberal, most support in interior semi-rural provinces, especially Germania, Thrace and Anatolia, as well as in the traditionalist areas of Hellas that do not bend towards Greek nationalism
"The Party" (name in Hebrew, as opposed to all above in Latin and/or Greek): Jewish nationalist, only relevant in Syria-Palaestina and Arabia Inferior

Election of 2758 AUC Results (MMP)
National Imperial Party: 579
The Left Bloc (Latin Left & Greek Socialist Movement): 391 (252 & 139)
Free Republican Party: 244
Party of Greeks: 95
Party of Faith and Traditions: 91
"The Party": 41

The election was a major defeat for the Left Bloc, which had formed a government for the first time in four decades after the 2754 elections. Support for the National Imperial Party rebounded after a series of confrontations between anti-monarchist factions in the government and Emperor Julian V which culminated in the Emperor dismissing the government two months early. In their usual display of nationalism, the Roman people backed their Emperor over the frustrated anti-monarchist left and punished the Latin Left particularly with their worst defeat in memory, losing all of their district seats in Britannia and Italia and all but one in Gallia. Still, after their stunning defeat in 2754, the National Imperial Party has not rebounded to historical levels, and it seems unlikely that the near-single party state that endured throughout the reign of the popular Emperor Julian IV will return now under Julian V.

Government: National Imperial Party and Party of Faith and Traditions form a coalition with the Free Republican Party tacitly supporting them

Also to note, Greek at this point is confined to Hellas, Anatolia, Syria-Palaestina and Aegyptus while Latin has wrapped around Greek and become the language of Arabia, Taurica and Dacia, all in the East, as well as a large minority in Aegyptus and about even with Greek in Syria-Palaestina.

[Verily]

No, I don't. Greco-Roman culture is fairly monolithic, and, assuming it survives, it easily overwhelms local Germanic, Slavic and Arabian cultures in the areas into which it would expand. Bear in mind that Britannia was actually one of the most staunchly loyal Roman areas under the Empire despite being at the fringes; generally, the fringes adopted Roman culture most ardently, while by the establishment of the Empire the internal Latin-speaking areas (Gallia, Hispania, Mauretania, Africa, etc.) had been fairly well assimilated.

[Verily]

To expand, there was really no sense of "regional" nationalism (perhaps best characterized in modern terms as ethnic nationalism, though even that doesn't quite capture the suggestion) in ancient Rome. Yes, regions occasionally revolted, but this was not the will of the local people, who by and large had no sense of community outside of family, village and empire, but local governors and/or generals (who were usually ethnic Latins and nearly always ethnic Italians until very late in the empire; more importantly they didn't share an ethnicity with the locals) simply trying to seize power for themselves.

Assuming a relatively stable democratic system can be established, there is no reason to suggest that such revolts would continue, though you might end up with a military-guided "democracy" as in Thailand, for example.

[Verily]

Some further notes:

The electoral system used is broadly MMP, with about half of the Senate elected on lists and the other half from local constituencies. There are actually two separate list-regions, Syria-Palaestina and everywhere else, as the Romans were concerned with the idea of Jewish nationalists representing everyone else, and, as historically, have some grudging respect for Hebrew civilization as even older than Greece. Syria-Palaestina has 154 seats; the rest of the Empire has 1087 seats. Seats have been added to the Senate more or less consistently since the final accoutrements of the old Senate were abolished in the Reform of 2519, which removed the vestiges of hereditary Senate membership as well as requirements of wealth to be a Senator and reduced the size of the Senate from several tens of thousands to (at the time) 501.

The Reform was enacted by Imperial fiat under Dorian III, and at the time was very controversial, with many of the hereditary Senators (who comprised about half of the chamber, with most of the rest at the time appointed by the Emperor) revolting and forming their own government that was not wholly defeated until the Battle of Sirmium six years later. (It should be noted that the Rebel Senate quickly fell into chaos due to competing interests among the powerful leaders, and although they did not disappear until after Sirmium, realistically they were doomed from the day they left Italy following the recapture of Ravenna less than three months after the revolt began.)