Compiler, Interpreter, Basically a PL could also be stack based but outputs LLVM IR or maybe JVM if you are feeling...(whatever people feel when they target the JVM), theres also QBE for backend output if LLVM is too much.
Multithreaded Web Server?
A sqlite clone? (Will cover databases, DSA and languages as well), Redis clone?
A Ray tracing or Path Tracing renderer?