A great plotline would be if Jessica came back as a sort of supernatural being herself. Maybe she was brought back by a powerful witch, but not fully human. This would create a whole new dynamic as Sam would have to deal with her new state. She could be on a quest to find out who brought her back and why, and in the process, she and Sam could rekindle their relationship in a very different way compared to before.
In some fanfictions, Jessica could have been saved by an angel at the last moment. This would set her on a path of learning about the celestial beings. She might become involved in the angel - demon war, trying to use her newfound knowledge and connections to help Sam and Dean. For example, she could act as a mediator between different factions of angels, using her charm and intelligence to prevent unnecessary bloodshed.
One possible plotline could be Kushina getting to train Naruto personally. Since she was a powerful kunoichi, she could pass on her unique skills and knowledge. This would make Naruto even more formidable at an earlier stage. Another plotline might involve Kushina's influence on Naruto's relationships. For example, she could have a big impact on how Naruto interacts with Sasuke, perhaps helping to prevent some of the rifts that occurred between them in the original story.
A possible plotline could also be Naruto's struggle with loneliness and how he overcomes it. He might start writing letters to his friends (even if he doesn't send them) as a way to express his feelings, and through this, he gains a better understanding of himself.
One possible plotline could be Sarutobi mentoring Naruto more closely. Since he's alive, he could pass on even more of his wisdom and jutsu knowledge to Naruto. For example, he might teach Naruto some of the more secretive fire - style jutsu that only he knows.
One possible plotline could be Harry trying to hide his wizarding past from his muggle friends and coworkers. For example, he might accidentally use magic in a moment of panic and then have to cover it up.
One interesting plotline could be Harry learning the pure - blood traditions from the Malfoys in a different way than he expected. He might gain a new perspective on the wizarding world's social hierarchy.
Well, in some Supernatural 13x17 fanfiction, there might be a plotline where a side character from that episode gets a more prominent role. For example, they could be the key to solving a mystery that was only hinted at in the actual episode. It could also be about how the events of 13x17 change the future trajectory of the characters' lives in ways that the show didn't explore.
One possible plotline could be Harry exploring his magical abilities in solitude. He might discover new spells or magical artifacts that he keeps to himself. Another could be him building a secret sanctuary where he can retreat from the wizarding world's chaos.
One interesting plotline could be the characters from different parts of Percy Jackson's world suddenly finding their lives intertwined in unexpected ways. For example, demigods from different camps having to team up against a new, powerful threat that targets all of them regardless of their origin.
One possible plotline could be Bella making new friends in La Push. Maybe she discovers a hidden talent within the community there.
One interesting plotline could be them sharing a flat in London after the war. They support each other through their post - Hogwarts careers, Hermione in the Ministry perhaps, and Harry as an Auror. There are little domestic moments like cooking together or discussing the latest wizarding news over breakfast.