Sherlock discovers that a while after his faked death, John suffered from a fairly severe case of stress-induced cardiomyopathy (https://en.wikipedia.org/wiki/Takotsubo_cardiomyopathy) (aka Broken Heart Syndrome*). This is something that John has taken pains to keep from Sherlock, uncomfortable with its implications. Sherlock is shaken by the discovery, and also baffled - at first thinking something or someone else must have been the trigger for John's condition. When he realizes that it was his own "death" that caused John's heart to dangerously weaken, Sherlock finds himself unable to suppress, or fully comprehend, his complicated reaction: The dominant three quarters of him is guilty and horrified, and wants to shake John for being so insane as to care that much about Sherlock. But there is another Very Not Good part of him that greedily hoards this hard evidence of his importance to John.

What happens from there is up to the author.

Alternate S3, or post-S3. Can even be set while John is still with Mary if the author chooses.

Johnlock please, though RST is not strictly necessary. Any rating is fine.

*I know Broken Heart Syndrome occurs mainly in women. But I can't find anything that says it happens exclusively to women, so.