Yes, it's quite normal. Fanfiction often offers unique takes on the 'Jujutsu Kaisen' universe, and it can enhance the viewing experience. It allows fans to explore different storylines and character developments that might not be in the official show.
Archive of Our Own (AO3) is a great place. It has a large collection of 'Jujutsu Kaisen' fanfiction. The tagging system there makes it easy to find specific types of stories, like those related to certain characters or plotlines.
One of the best experiences is seeing different interpretations of the characters. Fanfiction might explore a character's backstory more deeply than the anime. For example, it could give more details about Yuji Itadori's life before he joined Jujutsu Tech.
The Jujutsu Kaisen manga is indeed ending. It's been a great run, but all good things must come to an end. The final chapters are bringing closure to the characters and story arcs.