So, I was inspired by a theory that Silverlullabies on Tumblr had. Pretty much, they believed that the reason Voldemort's curse rebounded on him when he tried to kill Harry, wasn't necessary because Lily was willing to sacrifice her life for her son(because there's no way that Lily was the first mother ready to give her life for her child), but because when Snape begged Voldemort to spare Lily and the Dark Lord agreed(when he told her to step aside, actually intent in keeping his word), he accidently entered into a magical oath when he promised to spare Lily. But when he killed her, he broke that earth and so as punishment the next curse he cast turned against him and it would have ultimately killed him if not for the horcruxes.

Here is the exact quoting of the theory.

silverlullabies:

Okay so the part where Harry survived because Lily sacrificed herself has always bugged me because Lily couldn't have been the first mother in the history to have sacrificed herself for her child.

I was rereading the series and watching the movies when something clicked.

We all know how magical oaths are binding and breaking one could lead to being stripped of your magic and your death.

Well, I was reading when Snape figured out that Lily's son was the one that Voldemort was after and begged Voldemort to spare Lily's life. Voldemort actually agreed to spare her, promised Snape even- and meant it even because if you remember, he killed James Potter without second thought but told Lily to "stand aside" Like Voldemort actually did intend to keep his promise to Snape.

I think he accidentally created a magical oath at that moment when he promised Snape to spare Lily. So when he did kill her, he broke his oath and was killed as a result. So the next time he cast a spell or did magic, it was snap back and strip him of his life. It just so happened that the next bit of magic he performed was the killing curse at Harry. Instead of killing Harry it back lashed at Voldemort and would have killed him had it not been for the Horcruxes.

Now, inspired by this, I thought to tweak the theory a bit to make it work with my AU idea, where Lily and/or James lived to raise Harry. What if instead when Voldemort went to kill Lily, the magic oath he made with Snape did lag and wait until after the deed was done, what if the intent was enough and the curse backfired right then and there instead, leaving Lily with the scar but Harry with a piece of Voldemort's soul being that he was the purest thing in the room. Yes their are some holes in this theory, but it's enough to make this AU work.
If you're interested in a story using this theory as the backbone and where James and Lily live to raise their kid, then stick around. Now,here's the catch. It's a femharry story. I know not a lot of people like these type of stories so this might be the wrong story for you and I understand. Just please don't send me hate over it.

Either way, the first chapter will be arriving shortly. If you like the idea of this story, let me know. Encouragement and ideas are welcomed.