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?
A VR game where you take a snapshot or ghost of yourself performing some movement or throwing an item, which then repeats itself in the world. Then you can make more snapshots and string them together to create a Factorio-like game which uses the snapshot of those movements to assemble products.
E.g., ghost 1: pick up ore, throw ore -> ghost 2: catch ore, crush ore, throw crushed ore -> ghost 3: put ore into furnace -> ghost 4: pick up metal bar, throw metal bar, etc..
Then the player is running around building interactable buildings with produced resources and that oh so satisfying factory spaghetti starts forming.
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?