It's completely inappropriate. Such content violates the respect and integrity that should be maintained for the characters. Fanfiction should focus on positive aspects like character development, alternative storylines that are family - friendly or at least in line with general decency.
In Gwen Stacy and Miles Morales fanfic, the characterizations are really interesting. The writers can take the base personalities of these two characters from the comics or movies and expand on them. They might give Gwen more of a rebellious streak when she's with Miles or show Miles as being more vulnerable around Gwen.
Generally, Miles and Gwen's relationship in the comics is more about friendship and working together as superheroes. Romance isn't a major focus, so they don't typically 'get together' in the conventional sense.
Another great Gwen and Miles fanfiction idea is them teaming up to stop a powerful villain that neither could defeat alone. Along the way, they have deep conversations about their responsibilities as Spider - People and how they can make a difference in their respective worlds. This would explore their characters more deeply.
There's a fanfiction called 'The Web of Bonds' which is quite interesting. It takes a more emotional approach to their relationship. It shows how Miles and Gwen support each other through tough times and how their bond grows stronger as they face different challenges in the world of superheroes.
One possible storyline could be Gwen and Miles teaming up to take on a new supervillain in their city. They could start off not seeing eye - to - eye on how to approach the villain, but as they face different challenges during the fight, they learn to trust each other's instincts more. Another could be a story where they accidentally swap powers for a day and have to deal with the chaos that ensues while also trying to keep it a secret from their friends and family.
One great fanfiction is 'Parallel Hearts' where Miles and Gwen team up to face a new villain in an alternate dimension. It has great character development and action scenes.
Fanfiction.net is also a good place to find Gwen and Miles fanfiction. There are many different stories written by various authors, although the quality can vary. It's always a good idea to read the reviews and ratings before diving into a story.
Another one is 'Miles and Gwen: Unlikely Allies'. This fanfic takes place during a big city - wide crisis. They are forced to work together despite their differences at the start. The author does a great job of showing their dynamic and how they come to rely on each other.