As for features greek pagans could have in-game a conversion bonus against Pagans due to their semi-inclusive nature would be good. Maybe a system of traits like the hindus have to indicate various branches of Greek philosophy that your character might follow, so you could be a Stoic or a Cynic.
Here you go. Of course this is just taking the piss as this "Easter Egg" religion would have more mechanics than other faiths
HELLENIC
Still no raiding: We are civilized and all that jazz...
Female temple holders
Concubines
Olympic games: Grand tournament with new fluff text and lesser chance of death. Can be held every fours year but only during summer.
Orgies: Great feast, increased change to get Hedonist, Gluttonous, Drunkard Lustful and Syphilis traits
Vinalia: Summer fair
Sol Invictus: Solstice festival (as other pagans)
Patron Gods: Like Hindu Patron gods, different names naturally.
Pilgrimage: To Mount Olympus
Ludi Romani: Diviner organizes games. This would replace "Improve religious relations" ability and work as "Perform charity" that Muslim court imams have, except any related fluff text would be changed.
Pax Romana: New Casus belli, usable by king/emperor only. Subjugates a de-jure duchy.
Once there was dream of Rome: Ruler with Hellenic Faith can reform Roman Empire. Requirements are same as with Byzantines. If for some reason ruler is not yet Emperor when this happens, he receives titular "Roman empire"
Pontifex Maximus: Hellenic is already re-formed but by controlling three holy sites, religious authority of 50% and cost of 750 piety (or controlling all holy sites), Office of Pontifex Maximus is re-established. This functions as religious head for Hellenic faith and is a duke level title as with other pagan faiths, rather than Fylkirate.
Hellenic faith's holy order would appear and be of same composition as Brotherhood of the Holy Sepulchure (As for name of said holy order, no idea.)
Roman Culture & Retinue
Rather than having Roman retinue as such, vanilla retinues are renamed and to simulate Roman military culture, their effectiveness is increased by +5%. So Princepes retinue is +25% HI defense/+45% archer offense and so on.
Skirmish retinue is renamed "Velites"
Cavalry retinue is renamed "Equitees"
Defense retinue is renamed "Triarii"
Shock retinue is renamed "Principes"
Building: Roman barracks: +20 Heavy infantry, +10 Pikemen, +15% HI/Pikemen defensive