There's 'Yuji and Nobara: Unseen Connections'. This fanfic delves deep into their friendship that could potentially turn into something more. It has some really interesting scenarios where they rely on each other's strengths and learn from their weaknesses.
One common theme could be friendship. Given the nature of the characters in 'inu x boku ss', the fanfics might explore the deepening of friendships between the characters. For example, Nobara and other characters might have storylines that revolve around how they become closer friends and support each other.
I'm sorry I can't recommend specific fanfics directly as I don't have a list of them at hand. But you can try searching on fanfic websites like Archive of Our Own. There are usually a lot of fanfics about various anime and manga series there.
Itadori did indeed die in the manga. His death had a significant impact on the story and the other characters. It was a turning point that led to many new developments in the plot.