I've posted about it in another forum before, but I think there is much potential for a World of Darkness and/or Chronicle of Darkness Crossover:
Vampires - Take the "Vampire the Requiem" covenants. There are some good ways of including House Tepes into Ordo Dracul. For powers/bloodlines one can either use Vampire the Masquerade or Requiem (but I think Devils would profit more from taking some VtM ideas). Gives (un-grail-altered) Vamps a power boost that let them punch in the big leagues.
Mages - Take Mage the Awakening. The Technocracy doesn't fit DxD all that well so I would go for Traditions, Marauder and Nephandi (the modern Book of the Fallen ones). No more humans Mages need to do math and are less powerful, because that was stupid to begin with. A mage would become a real powerplayer even without a sacred gear (but it also comes with a bit downside in the fact of paradoxes). Archmages can rewrite reality, so yeah now they can go out and stand up to the supernatural.
Church - Is allied the "Lancea et Sanctum" covenant of Vampires and the Mages of the Celestial Chorus. That way they can send out teams exorcist with Mage or vampire backup and be a real danger, before they either had to send angels (which are a very limited resource as they can't make new ones) or exorcist with holy weapons (also very limited resource) to be a real danger.
Ghosts - Wraith the Oblivion ones, now they are a thing and have powers and organizations and stuff. What was Reapers job anyway in DxD, can't remember if there were any ghosts?
Werewolves - Not a big fan of the WOD Werewolves so I would go for "Werewolf the Forsaken" ones. Would give them new powers, an actual background and stuff.
But Werewolf comes with more than just Werewolves it also comes with a spirit world, basically objects, places and plants have spirits and if those get strong enough they can manifest in the spirit realm and if they grow stronger they can effect the material world.
Devils - DxD Devils are basically what Vampires are in other mystical genres, as in they can turn humans into on of their own and have a strict hierarchy with noble houses that all have different inherited powers. A such I think they could take some ideas of Vampire the Masquerade, especially
Roads. Those are "belief systems" and if you lose all your levels in your road (by doing things that go completely against anything you belief in) you lose your sense of self and are reduced to a raging beast, this would be a nice explanation for "Stray Devils" and on could add stuff like the seven deadly sins in there pretty easily, like have seven devil roads on for every sin. (Also can take inspiration from Vamp Clans/Bloodlines for new Devil Houses)
Other Stuff one could maybe use:
Fae - On can add either of the Changeling Brands, or non. Depends on if one feels like it.
Demons - Demon the Fallen ones, can posses humans, fight angels. Also adds Earthbounds as new antagonists.
The God-machine - What if the system of heaven isn't (as) broken? What if the angels are secretly manipulated without knowing it by the system they belief to be broken and it has a gained a conscience and is now following it own plans?
Mummies - Because why not? Also brings in cool Egyptian style stuff, which get neglected to much in favour for nordic and greek in most settings.
Kuei-Jin - A "vampire-like" species at home in east Asia.
Hunters - Human who can punch a supernatural being to death
The Abyss - the Dimension Gap but better