I thought of the SDRA2 fanfic idea that I will never use.

SPOILERS

Non-Despair/No Killing Game AU. 

Everyone is a adult because I said so.

So basically, it's Hibiki's birthday (also Kanade's) and Hibiki decides to bring her sister Kanade a cake in jail (Kanade is in jail for being a murderer and stuff) because it's Kanade's birthday as well. While visiting Kanade, Kanade knocks Hibiki (somehow she covered the camera in the room where you talk to people in jail) and disguises herself as Hibiki. She makes Hibiki look like her, and then she escape prison and goes to Hibiki's apartment because she figured where Hibiki lived when they were talking. (So basically Kanade is impersonating Hibiki. That's like the premise of the book.)

At some point Setsuka, Syobai, and Kokoro notice something off about "Hibiki" and later figure out it's actually Kanade. And at some point Kanade would basically use Iroha like the way she would use Hibiki as a puppet in the game. And then the real Hibiki (the one in jail) is trying to convince the wardens at jail that she isn't Kanade and that her sister is roaming free.

So basically it's Kanade impersonating Hibiki and then getting caught so she runs away from everyone or something. I don't know. I just thought of what would happen in the beginning.