Yes, there will be. J.K. Rowling has been quite engaged with this series and given its popularity, it's very likely she'll continue writing more Cormoran Strike novels.
As of now, there is no definite information regarding a new Cormoran Strike novel. The previous novels in the series have been well - received, which might suggest that there could be more in the future. But we have to consider that the author, J.K. Rowling, is involved in multiple projects. She might be exploring new ideas or working on other stories. The publishing world also plays a role. If the publishers see a high demand for a new Cormoran Strike novel and if J.K. Rowling has the time and inspiration, then it's very likely that we will see a new one. But until then, we can only wait and hope.
A great idea for Cormoran Strike fanfiction is to create a 'what if' scenario. For example, what if Strike never became a detective? How would his life have turned out? You could also focus on his family relationships more deeply. Write about his interactions with his half - brother, or how his mother's past influenced his own choices. And don't forget about the romantic aspect. A fanfiction about Strike and Robin's relationship developing in a more comical or tense way than in the original books could be really engaging.
One common theme is the development of their relationship. Fans often explore how they become closer and trust each other more. Another theme is mystery - they are in a world full of mysteries in the original story, and fanfictions often add new ones or expand on existing ones.