A possible plot could be Sam's relationship with Dean in this new form. Dean might be initially skeptical or even a bit scared of Sam's transformation. Sam could then try to convince Dean that he's still the same person on the inside. Also, Sam could be on a mission given to him by a higher - up angel. But he starts to question the motives of the angelic hierarchy as he realizes some of their actions might not be as noble as they seem. He could then use his new powers to try and change things from the inside.
Well, one plot could be that Sam's angelic transformation is part of a grand plan by a powerful being to tip the balance in a celestial war. Sam doesn't know this at first and is just trying to figure out his new existence. As he discovers the truth, he has to decide whether to go along with the plan or fight against it. He might also meet other characters from the Supernatural universe who are affected by his new status, like Castiel. Maybe Castiel could be a mentor to Sam in understanding his angelic powers, or they could have a conflict if their views on how to use those powers differ.
One idea could be an angel family dealing with a new threat from a powerful dark entity. The angels have to put aside their internal differences to protect the world. For example, an angel family that has a young and inexperienced angel who discovers a unique power that could be the key to victory but they need to train the young one fast.
Fanfiction.net is also a great place to look. There are many Supernatural fans who post their stories there. However, the search function might not be as refined as AO3. You may need to browse through different categories related to Supernatural to find the ones about Sam becoming an angel.
It could be that Sam is an angel who has lost his memory. The main plot then would be his quest to regain it. As he travels around in the Supernatural universe, he starts to have flashbacks and strange encounters. These could be clues to his past identity and his role as an angel. Along the way, he teams up with other hunters who initially are skeptical of him but later realize his importance. And his journey of self - discovery becomes the central theme of the fanfiction.
One idea could be to explore the backstory of the angel. Maybe they were once a high - ranking celestial being who fell from grace but still holds on to their sense of justice, which is why they carry the shotgun. Another idea is to have the angel team up with a human hunter in the supernatural world, and they face off against demons that are causing chaos in a small town.
One idea could be a character who discovers they have a latent ability to communicate with vetalas and must use this to stop an ancient evil vetala from rising. Another could be a vetala falling in love with a human and the challenges they face in their relationship within the supernatural world.
One plot idea could be that the reader, an angel, is sent on a mission to redeem Lucifer. Along the way, they have many intense philosophical debates about good and evil, which gradually lead to an unexpected bond between them.
A unique plot could be that their marriage gives them new powers. Since they are so closely bonded now as husbands, they can combine their powers in ways they couldn't before. For example, when facing a powerful demon, their united power makes them invincible.
One idea could be that Sam and Bella discover a hidden magical realm together while on a hike in the forest. They have to work together to find their way out and in the process, they learn more about each other's fears and hopes.
Maybe an angelic or demonic entity implants something in Dean that causes the pregnancy. Dean might initially be in denial and try to hide it from Sam. As the pregnancy progresses, he starts to experience strange powers or sensations. This could lead to a journey where they discover that the unborn entity is a key to saving the world from a great evil.
One idea could be that Naruto discovers he has the power to manifest angel wings. This new power could make him an outcast at first, as it's so different from his normal ninja abilities. People might fear or envy him. But then he uses these wings to save the village in a unique way, like flying above to spot an approaching enemy army early.