So, I am not sure if someone already did this, but this just came to mind when I was looking for new fanfics.
So, Cedric Diggory is a rather interesting character. He plays a huge part during Harry's fourth year and dies at the end of it. So, what if he gets sent back after his death? Now with the motivation to get even stronger, so that he can avoid his death.
To what time he gets sent back is entirely the author's choice. He can get sent back to his birth, his first year or even his seventh. There is enough information about him to write in his first year (1989).
Honestly this is just a really generic main character. (if you stick to the originals at least) In my opinion, this is not an awful Idea, but the character is way too generic.
Yknow, I think that merging his soul with a more evil character after getting sent back would also be a good idea. This would maybe solve the "too generic" part.
Off topic:
Man I probably should just rename this "book" to "Horrible Harry Potter Fanfiction ideas" from how many Ideas I have for HP Fanfics. It's just that there are no games that I like to play nowadays.