Hello ! I've never found a good fanfic that wasn't abandoned, so if you find one please tell me.
It needs to be well-written and with few grammatical mistakes.

 

I am searching for one as well. I read a couple of fics but the authors abandoned them not long after. I'll keep my eyes open for one and post it here. I am a bit disappointed that there was never a Fate Stay/ DC or Fate Stay/Marvel cross over. Shiruo as a hero in the JL would fit well

 

I read Fate's Gambit a while ago, which is a Harry Potter - Fate Stay Night crossover. It has an OP Harry, and a harem if I remember correctly, so nothing library worthy. But I enjoyed it enough to finish it so for me it's a guilty pleasure I guess. It's also crazy long, so if you enjoy it, it should last you a while

https://www.fanfiction.net/s/9586702/1/Fate-s-Gamble
When Zelretch conducts an experiment and the being known to all as Fate intervenes Harry Potter's life is changed irrevocably. Now being raised by those who don't fit the definition of 'Normal' it is a very different Harry that is unleashed on the world. Or is it worlds? Disclaimer: This is fan fiction, I only own the plot.
Rated: Fiction M - English - Fantasy/Family - Harry P., Rider - Chapters: 88 - Words: 927,883 - Reviews: 4,647 - Favs: 3,216 - Follows: 2,278 - Updated: Sep 16, 2014 - Published: Aug 12, 2013 - Status: Complete - id: 9586702

 

If you're still looking then I suggest Matou Shinji series.

Let me ctrl-c ctrl-lv my old recommendation.
Matou Shinji Series: Matou Shinji and the Philosopher's Stone, Matou Shinji and the Heirs of Slytherin, Matou Shinji and the Master of Death and Matou Shinji and the Broken Chains - It's crossover between Harry Potter, Nasuverse and the Assassins Creed, but it doesn't require knowledge of neither. Matou Shinji, protagonist, is unable to become magus and that makes him failure to his family. He would completely break, but he gets letter from Hogwarts. Wanting praise and attention he focus on giving his best to succeed at witchcraft. Will he obtain what he seeks? What will come with his friendship with Boy-Who-Lives? And why Hogwarts? Featuring two Voldemorts, rapist tanuki, Draco Malfoy working part-time at Saint Mungo and assassin creating elite school club. Nice blend of obscure elements or both settings with author original ideas. (Ongoing, regular updates)

Fic is not without its faults (and have some elements that give average DLP user heart palpitations) but it's probably the best you will get in terms of crossing those two worlds. I would rate series as a whole 7.5/10.
And I seriously need to write review for it. The current ones on this site aren't reflective of the first part, much less of the series as a whole.