I'm thinking I'd implement it as an unadoptable religion like in vanilla instead of a guild like in Shadow. The tag that makes it unadoptable is gone, so I'd just have to give every leader a -100 weighting towards it.
The religion itself would probably be founded by an event, although I might decide to let a Sheaim/Kuriotates-only ritual start it.
I'll probably bring the Wyrmhold back, and make having CotD have negative culture/yields in cities again. The Kuriotates and Sheaim would have Dragon Shrine UB's that more than compensate for this. (The Sheaim Dragin Shrine UB would probably allow some minor wyverns to emerge from their Planar Gates too.)
It will need disciples to spread the religion, and maybe priests and warriors too. I'm thinking these will all start Crazed. Sane people don't worship dragons, and those who worship dragons aren't all that loyal to their king or the established church. Since the Dragon cult is secretive, these units may be invisible or HN too.
Roar will probably keep its current scattering effect, plus regain it old effect of making CoTD units switch to the Dragon's side.
I may make the Dragon promotion provide blitz and flying. All the Dragons' strengths may approximately double. Dragon Slaying will be stronger too though, and available to any Hero.
Dragons will probably lose "Breath Fire" (which is very inappropriate for Drifa) and instead gain a Nuke Range. This will let them lethally attack at range with their full normal strength, and will make the whole map shake when they do so. Such attacks would cause diplomatic problems too. It will also make them unbuildable until the equivalent of the Manhattan Project is built. The Dragons themselves would be among the cheapest units in the game.
The equivalent of the Manhattan Project would be the last in a series of rituals. It would probably require Divine Essence, but the first ritual in the series would come as soon as Mysticism or even Ancient Chants. They would be available to any civ, only requiring CotD be present in the city. Each ritual would require that any civ have built the earlier ones. Some of the earlier ones may have effects like awakening some barbarian wyverns.