[LoneWolf23k]

I like the idea of making Angels and Demons into outer-dimensional beings, although I had the idea of taking elements from both Warcraft and Diablo in regards to the appearances and roles of each. And maybe derive some ideas from Starcraft's Protoss and Zerg.

The Angels would be a race of energy beings, made of light and sound and clad in metallic suits of armor and golden robes. Bound to a dimension called the High Heavens, they seek to bring order to the Multiverse and purge it of the Demonic Scourge.

The Demons are less a race and more an Infection, as all Demons were once mortal creatures, sentient or otherwise, that were infused with Fel energies and transformed, becoming immortal creatures bound to the Demons' home dimension of the Twisted Nether. Though ultimately loyal to the common cause of all Demonkind to spread and conquer, they are Chaotic and prone to backstabbing and betrayal against their own kind, with each Demon ultimately seeking its own benefits over others.

Neither one of them would actually have anything to do with human (or other) religion, though they wouldn't be adverse to exploiting human beliefs. Angels could alter their forms to appear like beautiful human angels, or become invisible and whisper to certain mortals, manipulating them to accomplish goals "In God's Name" for the Angels. While Demons would operate more directly, bargaining with opportunistic mortals or creating demon cults in remote wilderness areas.