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.
Well, in an anime mashup novel, one key element is the blend of different anime universes. For example, characters from a shonen anime might interact with those from a magical girl anime. Another element is the plot which often has to be carefully crafted to make these different elements fit together. And the art style, even in written form, might be influenced by the distinct visual styles of the animes being mashed up.
One novel approach could be using machine learning algorithms to analyze the context and requirements of the mashup project and recommend relevant APIs based on that.
One idea could be that Kira Yoshikage from JJBA somehow ends up in the Naruto world. Maybe his Stand ability could be misinterpreted as some kind of strange ninja jutsu. He might try to live his quiet life there, but his actions would surely draw the attention of the ninja. For example, if he starts using his Stand to kill people in the ninja villages, it would be like a new and unknown threat to the ninja, and they would have to figure out how to deal with it.