One possible plotline could be Gabriel coming back with new powers and a mission to set things right in the Supernatural world. Maybe he teams up with the Winchesters in a way they never expected.
Another plotline could be that Gabriel returns with knowledge of a greater threat approaching. He has to convince the other characters, who might be skeptical at first, that this threat is real and they need to act fast. This could lead to a lot of action - packed sequences as they prepare to face the unknown danger.
In some fanfiction, Gabriel's death could be a catalyst for a character's transformation. For example, Sam or Dean Winchester might become more determined in their fight against the supernatural after his death. They could vow to avenge him and in the process, face their own inner demons.
One interesting plotline could be Harry's discovery of his true parentage as the son of Gabriel. It might lead to a whole new set of magical abilities and a different relationship with the wizarding world. For example, he could have a unique connection to certain spells or magical creatures that are related to Gabriel's heritage.
Another plotline might involve Edward reuniting with his old friends and family. There could be emotional reunions and then they all team up to face a new, greater threat together. They might have to overcome past misunderstandings and grow stronger as a group. For example, Edward could find that his brother Alphonse has developed in unexpected ways while he was away, and they have to readjust to each other.
One possible plotline could be Kushina's reunion with Naruto and how she uses her skills and wisdom to help him in his battles. For example, she might teach him some new jutsu that only she knew. Another plotline could involve Kushina exploring the new ninja world that has changed so much since her time. She could interact with characters like Sasuke and Sakura, and have an impact on their development as well.
One possible plotline could be Naruto coming back with new and powerful jutsu. For example, he might have learned some ancient and forbidden techniques during his time away. This would make him even more of a force to be reckoned with in the Hidden Leaf Village.
One interesting plotline could be Lily Potter's adjustment to the new era she returns to. Maybe she has to deal with new magical technologies and social changes. Another could be her reuniting with old friends and family, like Harry, and how their relationships evolve.
A possible plotline could involve Naruto's return after having some sort of spiritual awakening. He returns to the village, and Sakura is the first person he seeks out. Their interactions then lead to a deeper understanding of their feelings for each other. This could involve flashbacks to their past experiences together and how those have shaped their current relationship.
One interesting plotline could be Harry Potter's return to Hogwarts as a teacher. He could pass on his knowledge and experiences to the new generation of students. Another might be his return to the wizarding world after a long absence and having to deal with new threats that have emerged in his time away.
One interesting plotline could be Peter Parker coming back to find that his city has changed a great deal in his absence. Maybe new villains have emerged and he has to quickly adapt to the new threats. Another could be his relationships with his friends and family being strained after his return, and he has to work to repair them.
A plot could revolve around the mystery of Gabriel's wings. They might have some hidden abilities that no one, not even Gabriel himself, is fully aware of. A young hunter stumbles upon a clue about his wings and teams up with Gabriel to uncover the truth. This could lead to a series of adventures and revelations about Gabriel's past and the true nature of his wings.