One common theme is friendship. Their relationship starts as a rather odd friendship between a brilliant detective and an ex - military doctor. Many fanfics explore how they grow to rely on each other more and more.
Maybe John was frustrated at Sherlock's eccentric behavior. Sherlock often acts in ways that are outside of normal social norms, like his extreme focus on cases and disregard for basic social niceties. This could have led John to lash out and call him a 'freak' in a moment of exasperation.
He might be initially shocked. Sherlock is used to John being more understanding, so this outburst could catch him off - guard. But then he could go into his logical mode and start analyzing why John would say such a thing.
One great sherlock x john fanfic is 'A Study in John'. It beautifully explores their relationship in a modern - day London setting. The author does an amazing job of capturing the essence of Sherlock's deductive skills and John's loyalty.
In many Sherlock fan fictions, Sherlock branding John could be a sign of his trust or a way to get a reaction out of him. It all depends on the author's imagination and the context they create.
In many Sherlock/John fanfictions, Sherlock's actions towards John show deep affection. For example, he often goes out of his way to protect John from danger. He might put himself in harm's way just to keep John safe, which can be seen as a sign of love. Also, his jealousy when John pays attention to others can be an indication. In some fanfictions, Sherlock is overly possessive of John's time and attention, much like a person in love would be.
The strong bond between them. Their relationship is a mix of friendship, mutual respect and a bit of mystery. In fanfics, writers can explore this relationship further, like showing how they rely on each other in different situations.
There could be a 'baby sherlock daddy john fanfic' that focuses on the emotional bond between them. For instance, in the story, John might be worried about Sherlock's future because of his different way of thinking even as a baby. It could also explore how Sherlock, despite being a baby, forms a special attachment to John. This kind of fic would be really heart - warming as it delves into their relationship at such an early stage.
Often, these fanfictions focus on the slow - burn aspect. Since Sherlock and John's relationship in the original series is platonic - ish at first, the fanfics gradually build up to the kiss. They also tend to explore the characters' emotions more deeply, like Sherlock's struggle to understand his own feelings.