I just re-read the final chapters of my own #YuGiOhGX fanfiction which puts the characters in an only slightly different universe / timeline.
It kind of dragged me down because while it does not have a bad ending in the classic sense, it is overall rather gloomy.
If you think that’s what you want to read right now, check it out:
https://archiveofourown.org/works/17611934/chapters/41519477
By the way while rereading it I thought that I’d completely messed up the continuity of the final duel because I rewrote it so often, but then found on closer inspection that I had just overlooked a sentence at the end of a chapter (which explains an important effect of one of the monsters) because my mind was already on the next chapter. All is fine, the story makes sense. But it’s still not really uplifting.