Man, this makes me think of those predictable heaven-and-hell politics I keep seeing. A lot of these stories center on a forbidden romance, obviously, with some star-crossed celestial and infernal lover pairing off against the wishes of both sides. The councils in both realms are always furious about the 'balance' being disrupted.
But honestly, the deeper conflict that actually interests me is the internal one. It's not just about defying orders; it's about these beings questioning the very nature of their existence. An angel who starts to feel the rigid dogma of heaven is cruel, or a demon who realizes they're more than just a vessel for sin. That internal crisis of faith, or lack thereof, is way more gripping than another big battle over a MacGuffin.
I've always found the 'prophecy' trope a bit lazy, though—the whole 'chosen one will end the war' thing. I'd rather see a conflict born from a simple, messy personal choice that escalates, not some grand cosmic destiny.