A great plot for a Spy Game fanfic could involve a mission within a mission. For example, the spies are sent to retrieve a valuable artifact, but it turns out that one of their own team members has a hidden agenda related to the artifact. This could lead to a lot of internal conflict and double - crosses. Also, you could explore the personal lives of the spies more, like their relationships outside of the job and how that affects their performance in the field.
One idea could be to create an alternate origin story for the main character in Spy Game. Maybe they were initially a hacker who got recruited into the spy world after accidentally uncovering a major conspiracy. Another idea is to have a cross - over with another popular spy franchise, where characters from both worlds have to team up to take down a common enemy.