Slitherine

The following is going to be like printing free money:

Make an RPG based on WW2. Complex "alternative" adventure; customization; emotional engagement (including loving interests); mindful progression; open world (or at least semi-open).

Picture this: a band of adventures roaming the landscape saving lives and finding secrets, driving panzers and improving their gear. Millions will buy it

Sadly I must tell you it has already been done

One title is perhaps old and hard to locate called Behind Enemy Lines.

Second option though is more current and design is based on Whitebox, a derivative of D&D game mechanics.

https://www.drivethrurpg.com/en/product ... n-whitebox

It is basically what you get when you design an RPG for a WW2 setting.