I've tried this. It's great.

[Mithras]

sorry about the lateness, busy life

Paganism/Heathenism overview:
Paganism in the context of the period isnt really a religion per-say, more a series of belief systems labelled rather crudely boxed into one area this makes trying attempts to define it a tricky proposition since it is all things to all people. It's plausable to be a monotheist or a Polythest or even a Deist within the 'religion' Here's a few of the sub divisions

Mystery religions: this is where Mithratic shrines in game come from, mystery rights are simular to christianity in their strong internal structure and deviation from social norms. They typiccly claim to have some sort of big secret which initates are let in on. This all goes on behind closed doors and is rather secretive. Notible mystery religions are the Eleusinian Mysteries, the Dionysian Mysteries, the Orphic Mysteries and the Mithraic Mysteries.

Note: many of these will overlap with christianity in the following centuries as it proceeds to dominate philisophical thought

suggested trait

Mystery intiate
+1 influece +1unreast +5% pagan conversion


Theurgy

Strongly linked with the neo-Platonists and technicly monotheistic in a simular manner to hinduism. Theurgy emphasises mysticism and unity with God,heavily involved in High ritualised magic in a simular manner to Hermetic mysticism (in my opinion).

suggested trait

+1 to moral, +1 to influence, +5% pagan conversion

Neoplatonism

also monotheistic and with a sort of dynamic panentheism, This is julian the apostates Religion of choice, stong emphasis on purity of thought, incorperates traditional Deities into itself. Ideas on Evil heavily influenced early christian Philosophy.

suggested trait:

+2 to management, +10% pagan conversion

Roman Imperial cult

The worship of Dead emperors is in decline within this period although the self-aggrandation of Christian rulers as semi-divine will continue for centuries. Essentially the idea that succesful emperors are promoted to Gods-rather looked down upon in well read circles if the play The Pumpkinification of Claudius is anything to go by

+10 to law,-1 management +10% pagan conversion

traditional Hellenic/Roman religion

Slowly in decline even prior to christianization and under siege from mystery cults and the slide in intellectual circles towards monotheism nontheless it finds supporters in high places such as emperor Diocletian and the Average Roman in the street

+2 influence, -1 to unrest, +10% pagan conversion

Heathenism

loose term for non-Roman pagans, covers a broad variety of Gods, belief systems and cultures: only relavent if charecter is roman.

+2 to unrest, -1 to influece

Suggested additional auxileries

pontifex maximus (title)
Vestal virgin
Mystery religion priest
Theurgist
Neoplatonist philospher
Heathen priest (barbarians)
Shaman(nomads, huns)
"az Isten kardja," the sword of God (Holy relic, Huns)

Suggested buildings:
total war standard Pagan temples: probably along the lines of: Mystery cult temple (EG Mithras), Traditional pantheon of some sort (EG:Jupiter or Mars)and an Imperial temple (Sol invictus or an dead Emperor), I'd be temped to add another tier to barbarian shrines as well
Lesser city shrines: possibly 2-3 single tier shrines to other gods within a pagan city or a building representing a temple district or whatever

Christian overview

Christianity is in an unusual position in the period one hand its traditionally been a messiac cult with apocalyptic overtones on the other its found itself heavily tied to the status quo-something it's traditionally railed against. Something has to give....


The only real change i'd note removing Arian heretic trait from Gothic and Vandal traits list and Replacing them with 'Nicean' (later be known as Catholic) I'd also be tempted to do the same with Pelgian in the Romano british but that would be a 'what if?' arc