Everything I write has some relationship with games, but the least-related would be MHS.
If you are aiming to expand your horizons and pick up a project that helps you grow as a programmer, there couldn’t be a better type of project than this.
In writing this software, I had to write a hex editor, disassembler, assembler, C compiler, DLL injector, memory-searcher, code-searcher, hidden-process finder, a debugger, a kernel driver, and more.
Writing these kinds of things will give you a great understanding of what is happening under the hood, plus as a game-hacking utility it allows you to look under the hoods of other games and see how they do what they do as well.
My tool is an all-in-one, but you could pick just one of these features and enjoy an enriching learning experience.
In any case, you should pick something that lets you grow as a programmer while still being related to what interests you most (necessary to keep your enthusiasm).
Want to learn a new language? Make an electronic dictionary.
Interested in space? Make a star-charting system or something to predict where planets will be at any time in the future.
Still a fan of old-school games? Make an emulator.