Would you consider adding a dog to the story? How humiliating would it be for one of these girls to be gang-raped first and then having the mouth and cunt offered to one or more dogs before being killed.

Making her suck it/them off, making her swallow doggie-cum. Putting whatever doggie-cum drips from her pussy put in her mouth too.

And insertion of a black scorpion tail into the vagina to see what happens or forcing the stinger into the clitoris. That kind of stuff.

Then killing her slowly by lowering her into a pond filled with piranhas… Inserting her vertically with her arms tied above her and her legs upwards hanging by the ankles so that after a minute or two the legs come off and hang separated from the body.

Might be difficult to implement in 3D, though