Too early to predict Battlefield 6 / VI ?
The next chronological, untapped era for FPS is the Korean War. It's the US, South Korea, and UN forces against North Korea, China, and even possibly some Soviet Russian interactions.
You'd still get WWII weapons, jet fighters, and the intro of the helicopter. It's a dynamic climate, too, which is very similar to the weather in say, NYC and Chicago or further north. So you can go wild with the weather. Geography is flat but also hilly and mountainous, and there are lots of marine interactions given the peninsula that the two Koreas are located in.
The beginning of it can be the Soviets fighting the Naz*s and kidnapping German scientists. It can play into the Manhattan project and how the secrets of the atomic bomb were stolen by a spy. Then lead the plot smoothly into the Korean War. It can be a smooth transition from BFV to BFVI.
Please make it happen!
EDIT: faction correction.