The JJBA manga is indeed finished. It had a great journey and wrapped up with a satisfying conclusion for fans. However, there's always the possibility of spin-offs or related works in the future.
There's also 'Diamond in the Rough'. This fanfic focuses on a unique relationship between the reader and a JJBA character. The writing style is very descriptive, especially when it comes to the Stand battles. It really brings the action to life and makes you feel like you're in the middle of the chaos.
Some JJBA fanfics that are worth reading are the ones that expand on the battles. They might add more complex strategies or new foes that the characters have to face. For instance, a fanfic could introduce a new stand user who has the power to manipulate time in a different way than Dio. Also, fanfics that give a different perspective on the existing story arcs, like seeing the events of Stardust Crusaders from an enemy's point of view, can be quite engaging.
One idea could be a crossover with another popular anime like Naruto. For example, Jotaro could meet Naruto and they could have a battle of wills and powers. Naruto's positive attitude might clash with Jotaro's more stoic nature.
First, you need to understand the JJBA universe thoroughly. Know the characters, their personalities, and the rules of Stands. For example, if you're writing about Jotaro, you have to capture his cool and aloof demeanor. Then, create a unique plot that involves the reader in a believable way. It could be that the reader has a Stand of their own and gets caught up in a battle against the main villains.
Well, it could be about the JoJo characters getting transported into the Warframe universe. They might team up with the Tenno to fight against common enemies like the Infested. For example, Jotaro could use Star Platinum's power along with a Warframe's abilities to take on hordes of foes.