The Lazarus Machine

Sir Harold Watson requires his younger brother John to marry for money. The wealthy husband-to-be? None other than Sherlock Holmes. Before the wedding can occur, Sherlock gets swept up in an investigation of random found body parts and strange letters addressed to him via Bow Street.

As John tags along on the investigation, getting to know his new husband, he finds himself in a London full of mad natural philosophers, rejected suitors, one overbearing brother-in-law, and more terrifying experiments than he could ever imagine.

Definite slash, though a slow build (but rated for the eventuality). Taking Sherlock Holmes back in time instead of forward.