In the context of Harry Potter fanfiction, 'Don't Fear the Reaper' could be a statement of defiance. The reaper can be seen as the symbol of the dark forces that bring death, like Voldemort. So, not fearing the reaper could mean that the characters are not going to be intimidated by the dark side's threats of death. It could also be a new way of looking at the cycle of life and death in the wizarding world, where death is not the end but just a part of a larger journey, and thus should not be feared.
It might signify courage. Just as in the original series Harry and his friends show courage in the face of death, in the fanfiction it could be an amplified version of that. Maybe a character is in a situation where death seems certain but they hold their ground.
The significance could be about the acceptance of fate. In the Harry Potter world, there are many prophecies and things that seem pre - determined. 'Don't Fear the Reaper' in fanfiction could mean that a character accepts whatever fate has in store, even if it seems as final as death. It could also be a way of showing that the character has grown beyond the basic fears that most wizards might have, and is now ready to face the ultimate unknown with a sense of peace.