One idea could be a fanfic that focuses on Cormoran Strike's early days in the military and how those experiences shaped his detective skills. Another could be a story where Robin and Strike have to solve a case in a foreign country, dealing with different cultures and languages. A third idea might be a fanfic that delves into Strike's family history and how it affects his present-day actions.
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.
You can try fanfiction.net. It has a large collection of fanfics from various fandoms, including Cormoran Strike. Just search for the relevant tags and you should be able to find some interesting reads.