Ask HN: What is the most useful thing I can develop in C++ and opensource it?

I am a C++ programmer with about 10 years of experience, have been reading HN for some time now.

I am based in Europe and looking for a job change (may be move to US). Lately, it seems like most good places in the US want you to demonstrate your skills through github before they would even consider you. I have always been one of the top programmers wherever I worked, but don't tend to program as much outside work and don't maintain a github account with any goodies.

My question is, is there something I can develop in C++ and put it on github that's actually useful for people and will help me get noticed? (I am also skilled in DB, Cloud technologies, python)