Ok, after nerding out for about an hour, I think I got it! 7 DLCs with 3 civs each, covering all of Roman history other than the unification of Italy and the stuff in the late 300s and 400s, which is getting into the mainline AoE 2 period. I do think another 3-civ DLC for the early Italian unification could be done (Romans, Etruscans, Samnites) but if they aren’t doing the Diadochi I doubt they will do Italian Unification, and from a marketing perspective the Punic Wars is way more flashy for sales anyway.
Punic Wars (Time Period 264 BC-146 BC)
Rome
Carthage
Iberians (will need to combine Celtiberians of the center and west with the non-celtic Iberians of the east coast of Iberia)
Roman-Greek Wars (This will be somewhat awkward as Rome’s wars especially with Macedon are tied in to the Punic wars (Rome not wanting Macedon to help Carthage) but I think is necessary to keep scope narrow and not add too many civs in one DLC. This will also include the wars in Illyria and the Roman-Selucid War, ending with the Achaean War and the annexation of mainland Greece. Time period 229 BC-146 BC. Could include a prologue of the conquest of the Greek cities in southern Italy, but that would push the start of the time period back to 280 BC
Illyrians
Selucids
Pergamon/Ionians (if no prologue is done, Representing the Kingdom of Pergamon and Greeks in west Anatolia as a whole, can be used to update the enemy faction in the Achaemenid portion of the Battle for Greece campaign as well)
Italiotes (Greeks in southern Italy, if the prologue is done)
First Bridge period (Better name pending) (Jugurthine War & Mithridatic Wars, time period 111 BC-63 BC)
Numidians
Armenia
Pontus
Triumvirate period (Third Servile War (Spartacus), Gallic Wars, Battle of Carrhae, Caesar’s Civil War, Second Triumvirate, War of Actium; time period 63 BC-30 BC
Ptolmaic Egypt
Parthia
Gauls
Imperial Period (Covering the Great Illyrian Revolt, Battle of Teutoburg Forest, Jewish Revolt, Roman Conquest of Britain time period 9 AD-84 AD)
Judeans
Germani (representing the western portion of Greater Germania, a distinction made by Romans)
Britons
Second Bridge period (Better name pending) (Domnitian’s wars in Dacia and Trajan’s wars in Dacia & Armenia, hitting Rome’s high-water mark 117 AD, as well as the Marcomannic Wars. Full time period 85 AD-180 AD)
Dacians
Suebi (representing the eastern portion of Greater Germania, a distinction made by Romans)
Sarmatians (Representing the Iazyges, with more utility later)
Roman Downturn (This DLC is more of an optional addition, but it brings us nicely to the transition into the base-game AOE2 timeframe. It covers the Sassanids taking over Persia from the Parthians in the roughly 220 AD, various very early (200s AD) Germanic & Sarmatian Barbarian incursions into Rome, and the “Crisis of the Third Century” (the rise and fall of the Palmyrene Empire & the breakaway Gallic roman Empire). This will end either with the abdication of Diocletian in 305, or the Battle of Milvian Bridge in 312 AD. So rough timeframe being 220 AD-312 AD. Perhaps simultaneously we can get a separate “traditional style” campaign for the base-game Roman civ)
Sassanids
Goths (The first incursion of Goths into the Roman Empire was the sack of Histria in 238, right in the middle of this time frame, and given their experience on the Steppes before that and their Scandinavian origin they seem perfectly acceptable to have as their own civ as opposed to being covered by the Germani)
Palmyra (i’m dubious on including this as a civ, because the story is interesting but it only lasted from 260-273, Palmyra could also just be represented by Sassanids)