The new story could be that Seilah is given a second chance at life by a mysterious force. She is reborn as a child in a village near the Fairy Tail guild. As she grows up, she has dreams and visions of her past life. She is curious about her past self but also feels a pull towards the good values she sees in the Fairy Tail members. She tries to find out more about her past while also learning the ways of kindness and friendship from the guild. Eventually, she has to face her past self in a showdown and choose whether to embrace her old evil ways or continue on the path of good she has started on.
One concept could be that Seilah discovers a powerful magical artifact in Fairy Tail's world. This artifact has the power to either destroy or save the world. She initially wants to use it for her own gain but realizes that if she does, it will bring destruction. The Fairy Tail members also learn about the artifact. They try to stop her but also help her understand the consequences. In the end, Seilah decides to work with them to protect the artifact and the world from other dark forces that want to misuse it.
Seilah could be cursed in the new story. The curse makes her lose her memories of being evil. As she wanders around clueless, the Fairy Tail members find her. They initially suspect her but then realize she has no memory of her past deeds. They decide to help her, and in the process, she forms bonds with them. When the curse is about to be broken and she might regain her evil memories, she has to fight against it to stay on the side of good.