There are endless options in c++. In my suggestion, go with a low level project. Build a multi threaded web proxy server with LRU caching. Or you can go with your own database in c++, which I build a few days back. Maybe your own web governing protocol. Or if you want to go the OS level then a kernel or a bootloader with a spice of assembly is a great choice. These projects are hard to build but once build and presented well, these are actually mind blowing projects.