Devil/Angra Manyu(only all the evils of humanity, psshaw) is weaksauce. We want Cosmic Eldritch Horror that Kotomine still ends up jacking because it's an excellent opportunity to make others suffer. I see no need to rehash BMW 1's plot outside of flashblacks/refereces. It wouldn't feel any less like a sequel.
A bumped up Angry empowered by all the evils of all humanities - now that could work.