Oh, that reminds me, I have a skeleton of a plot I started twice but never got to do
Basically modern fantasy, with the war between demons and angels (forces of Chaos and Order respectively rather than Evil and Good) coming to a head. Angels have a real hard time interacting with the world directly, so they've been safeguarding a 'pure' line, and now the time has come and they've kicked off the scion of the line's transformation into something that isn't human any more but isn't quite an angel. And I'm talking angels as they appear in the bible, not renaissance painting angels; fire and wings and interlocking wheels and eyes do not belong there. The scion didn't want this, they didn't want to be part of the war, and now they've got some serious body horror shit going on that they have no say in.
Meanwhile, a very clever second party made a very stupid decision and made a deal with a demon only to realize that, actually, they don't want to pay up. This is a very bad idea, and the demon is intent on collecting. They find two things in their search for a way out: the scion, and a ritual. The ritual is to create a familiar, binding a lesser Chaotic spirit (basically a baby demon, not fully formed) to the user. Importantly, the ritual fundamentally changes both parties; the caster is no longer entirely human, bound to the Chaotic energy of the spirit, and the spirit is conversely no longer fully Chaotic. The thing is, the familiar created is only visible and 'real' to the person they're bound to.
So, the second character approaches the scion with the ritual and outlines their plan: have themselves be bound to the scion, turning them both into something different, something neither the angels nor the demons can find easily. Only, the one being bound becomes a literal shadow to the world at large, only able to touch and interact directly with the scion.
So far a lot of potential partners haven't liked that part, or they want it to be pure sneaky smut instead of an interesting foundation for plot and character development. This outline is almost seven years old. I am and have always been entirely willing to play either character. I've started it twice but never even got to the ritual.