Castiel/Lucifer AU based on the comic book/graphic novel series by Kaori Yuki. Castiel as Alexiel, an angel who rebelled against God after seeing the cruelty he permitted and was punished by being constantly reincarnated as a human doomed to die a horrible death, and Lucifer as Nanatsusaya/Kira/Lucifer, who follows him through every lifetime. Alternately, you could switch their roles or have one of them in the place of Jibril/Sara, who is intended to be the "guard" of Alexiel, making sure that fate plays out but incidentally falls in love with one incarnation. Bonus points if you hurt me by having the Nanatsusaya or Jibril character "break the cycle" and defy God-- that makes sense if you've read the comics.