A popular one could be 'Merged Realms'. In this fanfic, the Hogwarts and the Phantomhive Manor settings are blended. It often features Harry and Ciel working together to solve a mystery that involves both the wizarding and the demon - filled worlds of their respective stories.
Fanfiction.net is another good place. There are many talented writers who post their Harry Potter Kuroshitsuji fan creations there. However, the search functionality might be a bit less refined compared to AO3. But if you dig a bit, you can still find some great stories.
These fanfics usually focus on giving Izuku a younger appearance and exploring the new possibilities and challenges that come with it. They might change the dynamics of his relationships and his growth journey.
The main plots usually revolve around Thor being de-aged and his adventures in this new form. Maybe there's a quest to regain his original age or he has to adapt to a younger body in the modern world.
Well, 'Joseph's Second Chance' is a notable one. It has a well-crafted story where the de-aged Joseph faces new challenges and grows in interesting ways. The author does a great job of maintaining the essence of the character while exploring this new age dynamic.
There's this one called 'Hamilton's Youthful Adventures' that's quite popular. It's got a fresh take on Hamilton's early years and the challenges he faced.
In such fanfics, you might find Naruto dealing with the expectations and responsibilities of being a ninja at a younger age. They might also play with the idea of time travel or alternate timelines, adding an extra layer of excitement. The character development can take unexpected turns too.