It could be Zeref. If Natsu goes back in time, Zeref might have different plans or be in a more powerful position at that time. He could be the main obstacle for Natsu to overcome, especially if Natsu is trying to change things related to the dark magic that Zeref wields.
One interesting plotline could be Natsu going back in time and accidentally changing something that has a huge impact on the future of Fairy Tail. For example, he might prevent a key member from joining the guild, and then he has to figure out how to fix it.
Well, in this kind of fanfiction, Natsu might use his knowledge of the future to prevent some tragedies. For example, he could stop certain battles from happening or save characters who died in the original timeline. Maybe he would also try to change the power dynamics within the guild earlier on, making Fairy Tail even stronger from the start.
She could meet her younger self. This would be an interesting encounter as she could offer advice and guidance to her younger version. Maybe she could help her younger self to be less afraid of her powers.
Natsu's character could develop a greater sense of responsibility. Going back in time, he realizes the importance of his actions on a whole new level. He might become more cautious in his decisions, as he knows the consequences they can have in the future.
One common theme is self - discovery. When Natsu goes back in time, he often learns more about himself, his powers, and his past. For example, he might find out things about his family or how he came to be who he is.
Gray is also a popular character to be paired with Natsu in these fanfictions. Their friendly rivalry can take on new dimensions when Natsu travels back in time. They could end up working together in unexpected ways in the past.
Some common themes could be time travel adventures. Natsu might go back in time to prevent a disaster in the Fairy Tail world. Another theme could be self - discovery. As he travels back, he learns more about his past and his true potential.
He could change the course of major events. For example, he might prevent the rise of a powerful villain. This would have a huge impact on the entire galaxy. Maybe he could save some characters who died in the original timeline.
One interesting aspect could be that Luke goes back to a time when the Sith were just emerging. He could try to root out the Sith threat before it became as powerful as it did. He might have to be very careful not to disrupt the timeline too much, but his Jedi wisdom could be a great asset in this time - traveling adventure.