Oh, I've got a good one, if we like the idea of making Mercy totally batshit insane for the sake of erotica.

As a sequel to the Tracer story, maybe Mercy is trying to experiment on the use of her healing technology.... and decides to forcibly volunteer Tracer and D.Va whom she hangs and keeps alive using her technology so they are hanged for insane lengths of time while forced to remain conscious and struggling while fighting for air.

Out of scientific curiosity, she manages to trick Widow into captivity so all three necks can feel the noose's grip for the very maddest of science.