In Fairy Tail fanfiction, Deku could be a very interesting addition. He might bring his heroic and determined nature from My Hero Academia. He could start as an outsider, much like he was in the beginning of his own story, and have to prove himself to the Fairy Tail guild. His unique power, One For All, would also be an exciting element to incorporate, perhaps having different interactions with Fairy Tail's magic system.
Toge Inumaki is important too. His special cursed speech ability makes him stand out. Also, the principal of the school, Masamichi Yaga, is likely to be important as he is in a position of authority and has an influence on Gojo and the other students. There could be other characters like the members of different clans that also play roles in the complex web of the story.
One great Gojo fanfic is 'Gojo's Ascent'. It delves deep into his character, exploring his powers in unique ways and his relationships with other characters. The story has a great plot that keeps you hooked from start to finish.