I'd recommend 'Sam's Journey into the Vampire Realm'. This fanfiction details Sam's accidental entry into the vampire realm and his struggle to find his way back home while also dealing with the various vampires he meets. It's a great story that combines adventure and the allure of the vampire genre. There's also 'Vampire Love and Sam' which delves into a romantic relationship between Sam and a vampire, exploring the challenges they face.
In Sam is a vampire fanfiction, the theme of forbidden love is quite common. If Sam gets involved with a vampire, their relationship is likely to be seen as taboo. Additionally, the search for identity can be a theme. Sam may be questioning who he is in relation to this new vampire - filled world he has entered, and how he fits into it all.
One possible analysis could be that it might explore the darker sides of the characters Dean and Sam in a vampire - themed scenario. Maybe it delves into how they deal with the vampiric nature, whether they resist it or succumb to it. It could also be about their relationship dynamic changing due to the vampiric influence.
One common theme could be the struggle between good and evil within the characters. Even as vampires, Dean and Sam might still have remnants of their former moral compasses. Another theme could be isolation, as they may be shunned by their human friends and allies due to their vampiric nature.
Sam could be just an ordinary guy who loves exploring the woods. He might be a nature enthusiast who spends his free time in the forest, and one day he just happens to come across Bella.
Dean is almost always going to be in these fanfictions when Sam is gagged. Their relationship is central to Supernatural. Rowena could also be a character. She might use her magic to either gag Sam for her own purposes or to help him if she has a change of heart. And Gabriel. He could turn up and use his trickster abilities to either put Sam in the gagged situation as a joke or to get him out of a more serious one.
Carly would probably be the first one to take care of Sam. They're best friends, after all. She'd bring her soup and make sure she has enough blankets.
One possible fanfiction could be about Sam and Mikaela starting a family. Maybe they find an abandoned Autobot baby. Sam, with his experience with the Autobots, wants to take it in and Mikaela is initially hesitant but is won over by the baby's cute beeps and whirs. They then face the challenge of raising a baby that has unique robotic needs while also trying to keep it a secret from the world.
Yes, they can be considered soulmates in certain fanfictions. Fanfiction gives authors the freedom to explore different relationships. Writers might emphasize their shared experiences in the werewolf/vampire world, or create a backstory that binds them together on a spiritual level, making them seem like soulmates.
I really liked 'Carly, Sam and the Mystery of the Old Mansion'. In this fanfiction, Carly and Sam find an old mansion and start to explore it. Along the way, they encounter strange things and have to rely on each other. It has a touch of mystery and adventure, which makes it a really engaging read.