One popular trope is the 'fish out of water' situation where Benedict Cumberbatch's Sherlock is thrust into a situation created by a fanfiction writer that's different from the show. For example, being in a modern-day high school setting while still having his brilliant detective mind. Another common one is the 'hurt/comfort' trope, where Sherlock gets injured during a case and his friends, or sometimes a new character from the fanfiction, take care of him. And then there's the 'alternate universe' trope, like a Sherlock in a steampunk world or a world where magic exists.
Well, in Sherlock fanfiction involving Benedict Cumberbatch, a major trope is the 'enemies to lovers' one. This could be between Sherlock and a new character or even a different take on his relationship with Moriarty. There are also time - travel tropes. Imagine Sherlock traveling back in time to solve a historical mystery. And the 'found family' trope is quite popular too. Sherlock, who often seems aloof in the show, finding a group of people who become like family to him through some wild fanfiction plot.
The 'mystery within a mystery' trope is often seen. So, not only is Sherlock solving the main case in the fanfiction, but there's also a sub - mystery related to his own character or past. Also, the 'forced proximity' trope can be interesting. Maybe Sherlock and Watson are stuck in a small space together during a case, which leads to some intense character interactions. And the 'power - swap' trope, where Sherlock loses his deductive abilities and has to rely on others while someone else takes on his brilliant mind for a while.