Codex I.a
Honorius was an attractive chap wasn't he?
The policy of the emperors and the senate, as far as it concerned religion, was happily seconded by the reflections of the enlightened, and by the habits of the superstitious, part of their subjects. The various modes of worship, which prevailed in the Roman world, were all considered by the people, as equally true; by the philosopher, as equally false; and by the magistrate, as equally useful. And thus toleration produced not only mutual indulgence, but even religious concord.
The superstition of the people was not embittered by any mixture of theological rancour; nor was it confined by the chains of any speculative system. The devout polytheist, though fondly attached to his national rites, admitted with implicit faith the different religions of the earth. Fear, gratitude, and curiosity, a dream or an omen, a singular disorder, or a distant journey, perpetually disposed him to multiply the articles of his belief, and to enlarge the list of his protectors. The thin texture of the Pagan mythology was interwoven with various but not discordant materials.
-Edward Gibbon
(I'm going to try and order information about the mod into codexes, in order to keep track of what's going to be changed, of course nothing is set in stone, and it's all up for discussion and change).
Roman Religion and Culture
My intention for twofold, I want to accurately represent the historical differences at the time, but I also want to have a system whereby this can change (after all this is the time of great germanic and steppe migrations), by a fluid process. Melting pot events are a bedrock of how cultures will change over time (Angles, Jutes and Saxons into Anglo-Saxons for instance).
Roman Culture will consist of a Latin Group, and various Roman cultures based upon geographical area. So Romans in Britain will be Romano-Britons, ect. A single Roman culture would have meant that the Melting pot events could have had some very unintended consequences.
My intention at the moment is to have a large number of historical and possible cultural synthesis possible. For instance Germanic Franks who settle in Gaul (Culture Gallo-Roman) will, over time, change the culture to Latin Franks. Same goes for the Lombards (Italy) and Goths (Spain). However say the Vandals were to stay in Hispania, then they'll be able to change to a new culture too, it won't be completely historically deterministic.
...
Now Religion. Paganism is not in a good state by the death of Theodosius, it's political capital is at an all-time low and outside rural areas and the Senatorial elites, it's a dying religion. Now, this is where the Old Gods DLC could be great. You could try to be another Julian the Apostate and bring paganism back to prominence by reforming it (which is one of the abilities of the DLC).
Christianity, on the other hand, is on the rise. It's got a strong power structure in the Pentarchy and good organisation in the Church Apparatus. However it's also starting to have trouble with heresies ( Arianism, ect.), and needs Imperial support to stay afloat. For instance, Britain's Christian population were almost entirely Pelagian at this point (an interesting synthesis of traditional Druidism and Christianity).
There will be a selection of smaller groups who never fully joined with the Roman Pantheon (Manichaeism is a good example for this). However mystery cults will be included within the Roman Pagan Pantheon, for the most part they ended up part of mainstream Roman Paganism (and for gameplay reasons, I don't really want a Mythra crusade for Britannia going on, that's what Lux is for).
Thankfully I have a Series of lectures on tape called "Fall of the Pagans and the Origins of Medieval Christianity" which is very helpful on this subject, so I should be able to do it justice.