It's a great list. Would be a good to keep it as some live list so we can help evolve it over time.

I'd add two ideas:

Build lightweight memcached - it requires mixture of basic algorithms as well as good system design practices - it's a "simple" problem, but requires to think well from multiple angles - invalidation, concurrency, memory management...

Build your own docker - it's not as complex as it might sound - gives opportunity to understand basics of OS programming, which most engineers nowadays don't really use in day to day jobs