One plot idea could be Sam going back in time to prevent a major tragedy in his family's past. For example, he could try to stop the demon from killing Mary Winchester, which would change the entire course of his and Dean's lives. Another idea is that he goes back to a time when hunters were just starting to organize and he tries to influence the development of the hunting community for the better.
He could meet his grandparents. Meeting them would give him a deeper understanding of his family history and the roots of the Winchester family's connection to the supernatural. Another interesting character would be young John Winchester. Sam could see what his father was like before all the tragedies happened and maybe even influence him in a positive way.
Often, Dean will go back in time and accidentally change something small that has huge ripple effects on the future. Like he might save someone who was supposed to die, and then that person ends up causing a lot of chaos in the future that Dean has to fix. This creates a cycle of trying to set things right again. Another plot twist could be that when he goes back, he meets an alternate version of himself or his loved ones that are very different from what he knows, and he has to figure out how to deal with them.
I really liked 'Sam Winchester's Time - Twisted Journey'. In this fanfiction, Sam accidentally travels to the future and sees a world devastated by a powerful demon. He then has to find a way to travel back and stop it from ever happening. 'Time Traveler Sam' is also a good read. It involves Sam traveling to different points in time to collect powerful artifacts that could help in the fight against the apocalypse. And don't forget about 'Sam's Time Warp', where he gets stuck in a time loop and has to figure out how to break it while also dealing with various threats.
Well, in some fanfictions, they go back in time and accidentally interfere with the lives of their younger selves. This creates all sorts of chaos as they have to deal with the younger Sam and Dean's reactions to seeing their future selves. Or perhaps they go back to a crucial moment in the history of the hunter community and end up changing the rules and relationships within it. It could also be that they go back in time to a specific era like the Wild West and have to deal with supernatural threats that were prevalent during that time.
The main characters are Sam and Dean Winchester, of course. But in some fanfictions, Bobby Singer might also be involved. Since he was like a father figure to them, he could be the one guiding them through the time - travel process, or he could be there in the past waiting for them. Also, the characters of their parents, Mary and John Winchester, might show up if they go back to a time when their parents were alive.
Sam Winchester is a very popular character in the Supernatural fandom. Time travel fanfiction about him is popular because it gives fans a chance to explore different timelines. Fans can imagine Sam in historical settings or in the future. It also allows for creative interpretations of the Supernatural lore. For instance, Sam could go back in time and learn new spells or meet ancient supernatural beings that could impact his fight against evil in his own time. Moreover, time travel stories often involve a lot of mystery and adventure, which are elements that attract readers.
Typically, in bella and sam winchester fanfiction, there is a strong focus on their relationship. It could be a slow - burn romance where they gradually fall in love while dealing with the perils of the supernatural world they inhabit.