One idea could be a character who discovers a hidden power within themselves that changes their pre - determined path. For example, a character who was meant to be a simple peasant but then realizes they have magic that can make them a powerful leader. Another could be a time - travel plot where a character goes back in time to change an event that was supposed to lead to their downfall. Maybe a character was fated to die in a battle, but by changing a small decision in the past, they rewrite their entire future.
Well, a great plot could involve a prophecy. Suppose there was a prophecy about a character's destiny being to bring destruction. But the character, through their own free will and the help of new friends they make along the way, decides to change that destiny and instead bring peace. Or perhaps a character from a different dimension enters the world of the fanfiction and their presence causes a shift in everyone's destiny. This could lead to all sorts of new alliances and conflicts as the characters adjust to the new order.
A possible plot for 'a change in destiny fanfiction' is that a character is given a second chance at life. They had a tragic end in their original destiny, but now they are reborn with the knowledge of what went wrong. They then set out to change their relationships, make different choices in their career, and overall avoid the mistakes that led to their downfall. This could involve them mending broken relationships, learning new skills, and standing up to the people or circumstances that previously held them back.