Okay this going to be a bit wordy and weird so bare with me. I want an AU where after Sherlock's fall John becomes a Dexter-ish killer. Basically, a serial killer who only kills other serial killers and bad people in general. He does this because he's torn between the parts of him who want danger and to feel useful again, with maybe a little bit of revenge on the side, and the parts of him who still want to help out the Met and be moral and good. When Sherlock comes back he can almost instantly see John's handy work in all the bodies appearing in London but he doesn't want to believe it. Author can go wherever from there.
Bonus: John has become a bit unstable and actively believes that Sherlock would be proud of him and/or is certain the bodies will bring his supposedly dead flatmate back to him.
Extra bonus: Lots of focus on morals and inner dilemma!