Comment by mcnnowak

Try playing Dungeons & Dragons or Pathfinder (or any number of other systems.) Pathfinder is super deep and complex.

There's also an entire community of people who play Table Top Role-Playing Games (TTRPGs) solo and use the outcomes of their play reports to blog or write fiction.

Also, the tooling around these games is very interesting if you want to build an app: Crafting calculator? Generative hexcrawl maps? Random tables? Statistics tools for dice rolls?