Tech itself. The hard skill. A lot of people realize that soft skills are important, but IMHO they are now the mainstream now. Everyone, including new graduates, know that. But many people are ignoring the hard skills, or simply do not have the time to dive deep into it (we can practice soft skills in time slices, but cannot do that for hard skills).

Some obscure hard skills from top of head:

- Replicate what John Carmack did for Commander Keen in DOSBOX (or a more accurate emulator, or a physical computer) against the 80286 architecture.

- Write a complete 2D game engine, or a complete but simple 3D game engine.

- Write a complete compiler, with code generation for a real architecture instead of a VM, for a simple language, such as Minimum BASIC.

Disclaimer: This is just my personal opinion. I know these projects are probably of minimum value, but could be great learning projects. They either force you to certain resource constraints (like Project 1), or read specifications and manuals (like Project 3), or learn to structure and organize larger projects (like Project 2).