My all time favorite trope is the fake/pretend relationship for a case that leads to realization of actual feelings. I've seen it done a few times where they have to pose as a couple because our bad guy of the week has been targeting homosexual couples who visit a specific resort/hotel/other location of author's choice. But this time, our bad guy of the week has been targeting heterosexual couples. Sherlock somehow convinces John to act as a couple for the case. Only John is unaware who the woman he will be pretend to be dating is. He argues that Sherlock can't drag Molly or any other woman they know into such a dangerous case. So he just waves John off and says he'll be the woman then. John thinks he can't be serious, and thinks at the very least it will at least be hilarious to see the detective look like someone's 12 year old sister. But Sherlock, master of disguise that he is, actually can be a VERY convincing woman, despite John's doubts. And apparently everyone else is convinced that John now has quite the lovely lady around his arm (bonus if you can include Mrs.Hudson's/Lestrade's/Molly's/anyone of your choosing's reaction to being told that it's actually Sherlock).

TLDR: John and Sherlock go undercover as a couple, only Sherlock has to pretend to be a woman, and John can't help but think how unfair it is that the detective looks amazing no matter if he's a man or a woman.