Christopher Nolan is renowned for his intricate plots, meticulous attention to detail, and groundbreaking narrative structures. Given his versatile filmography, which ranges from psychological thrillers to science fiction and historical dramas, the possibilities are vast. Here are some suggestions for themes or genres Nolan could delve into, based on his filmmaking style:

1. A Time Travel Historical Epic: With Nolan’s penchant for playing with time (as seen in “Inception”, “Interstellar”, and “Tenet”), he could tackle a time travel film where characters navigate major historical events, perhaps to prevent a future catastrophe.

2. Mystery Noir: A neo-noir detective story set in the 1940s or 1950s, combining the intricacy of “Memento” with the darkness and aesthetic of classic noir films.

3. Virtual Reality: Delving into a future where the lines between reality and virtual reality blur, challenging perceptions of what is genuine and what is a simulation, reminiscent of the layered realities in “Inception.”

4. Epic Fantasy: Nolan could bring his unique touch to the fantasy genre, reimagining classic myths or legends, and presenting them with his signature nonlinear narrative style.

5. Cosmic Horror: Given his foray into space with “Interstellar”, Nolan could explore themes of cosmic horror, touching on the insignificance of humankind in the vast expanse of the universe, drawing inspiration from writers like H.P. Lovecraft.

6. Biographical Film: Nolan’s realistic approach might lend itself to an intimate yet grand biographical film about a historical figure, presenting their life in a nonlinear narrative, much like he did with time in “Dunkirk.”

In the end, whatever subject or genre Nolan chooses, it’s almost a given that he would bring his distinctive flair to it, making it a memorable cinematic experience.