I am looking to start working on an Open-Source C++ side project and am in need of good ideas.

I am quite experienced writing in modern c++ after about 2 years of writing c++ as a full-time job.
I am mostly experienced in writing for the Windows environment (so VS is my IDE).

Any suggestions for C++ libraries that you wish someone would write but no one did yet
(or did it badly or too long ago)?

I don't mind joining an existing project so long as it's not too big yet, but I would rather start a new one. either way I would love it if someone were to join me :)

What about a library for creating ebooks - .epub or .mobi.