What’s a challenging programming project I can grow over time?

Thanks for the suggestions. I’m leaning toward making a game so I can start with a small playable version and build it up gradually rather than planning the whole thing in advance.

A multiplayer game or online tabletop-style platform could teach you a huge range of skills: client-server design, matchmaking, persistence, synchronization, security, moderation, and deployment. Start with a tiny playable prototype and add features only after the core loop works. You don’t need impressive graphics; a well-designed system with reliable gameplay can be a much better engineering project.

The best long-term project is usually something you personally care about or would genuinely use. Think about your hobbies, recurring frustrations, or repetitive tasks and build a tool around one of them. Motivation matters more than picking the most technically impressive idea, because a project you care about is much easier to maintain when the difficult parts show up.

Build a small game inspired by something familiar, then keep expanding it instead of trying to design a massive project from day one. For example, start with a basic maze game, then add smarter opponents, procedural levels, multiplayer, 3D graphics, mod support, or even VR. A familiar starting point gives you a clear first milestone while leaving plenty of room to learn about architecture, performance, networking, and content pipelines.

Another strong direction is a game randomizer or modding tool for a game you know well. Begin by reading data, changing a few items, or generating alternate challenges. Later you could add a plugin system, a desktop interface, validation, seed sharing, multiplayer coordination, and automated testing. That kind of project combines algorithms, file formats, tooling, and user experience without requiring you to build an entire game engine.