One common theme is identity. Simon becoming a vampire changes a lot for him, and many fanfictions explore how he comes to terms with this new identity. Another theme is love and relationships. His relationships with other characters, especially romantic ones, are often explored.
One really good 'Shadowhunters vampire Simon fanfiction' that I've come across is one that delves deep into Simon's internal conflict. He's a vampire, but he still has his human side and the values he held as a human. This story shows how he tries to balance his new vampiric nature with his old self. It also has some great interactions with the other Shadowhunters characters, like Clary. The author does a wonderful job of portraying Simon's complex emotions and the challenges he faces in this new life.
In some fanfics, Simon and Clary's love is a slow - burn type. They start as friends, but as they fight side by side against demons, they gradually realize their feelings are more than just friendship. For example, there could be a scene where Simon saves Clary from a particularly powerful demon, and in that moment of relief and gratitude, they share a kiss that changes everything between them.
I really like 'Mixed Realms of Blood and Magic'. This fanfiction takes the best of Vampire Academy's vampire lore and Shadowhunters' angel - demon magic system. It has a great protagonist who is trying to find her place in this combined world of vampires and shadowhunters. There are some really intense fight scenes and lots of character development throughout.
First, you need to study the lore of both Vampire Academy and Shadowhunters. Know the rules about vampires, dhampirs, shadowhunters, etc. This will make your story more believable. For example, in Vampire Academy, the relationship between vampires and dhampirs is very specific.
Another scenario could be that Clary has to choose between her duties as a Shadowhunter and her love for Simon. She loves the Shadowhunter world and wants to protect it, but she also loves Simon deeply. In the fanfic, she might find a way to balance both, which is a great exploration of their love.
One interesting Simon vampire fanfiction could be about his origin as a vampire. It might explore how he was turned and his initial struggles with his new identity. Maybe he was turned by a powerful ancient vampire against his will, and he has to learn to control his bloodlust while still maintaining his human relationships.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions from various fandoms, including those related to vampires. Just search for 'Simon vampire' on the site, and you'll likely find a good number of relevant stories.
Another theme is his place in the vampire world. He has to figure out where he stands among other vampires, whether he'll follow the rules or rebel. Some fanfictions might focus on his interactions with more powerful vampires and how he tries to assert himself or find his own path within the vampire hierarchy.
You can start by looking on popular fanfiction websites like Archive of Our Own (AO3). There are often many well - written Vampire Simon Lewis fanfictions there. Just use the search function and type in 'Vampire Simon Lewis' and you'll get a list of relevant stories.