I'm thinking Dorian is the sort of guy who might show what Tevinter could be like if they let go of slavery and became the society Andraste's follower the Archon meant it to be. Being a worshiper of the Black Divine would be interesting and fit with the idea the game is partially about faith and what people believe in.
We haven't really seen much about the ways that the Imperial Chantry differs from the southern one, either, apart from having a male clergy. It's been mentioned that Tevinter's Chant has been modified in some way to allow for a magocracy, and I'd like to explore how Dorian reconciles his use of magic with what the other Chantry says.
There's a lot of cool stuff about apocryphal Chant of Light verses, which seems to be a big focus of Inquisition regardless.
(And now that I say that, I realise that woah the Byzantine references really aren't subtle.)
World of Thedas didn't give any indication that Tevinter's Chantry would have an issue with his sexuality, so I could quite easily see Dorian being a devout Andrastrian. Like you said, it's another perspective on faith that we haven't seen in the game before.