Wow, just last week we were saying how much we get questions like these. Read the questions and answers on that question
Suggest OOP project in C++[
^]. You'll find that it mostly depends on your own knowledge and interests. Google around too, can be a great source of inspiration. I just found
this[
^] for OpenGL. The techniques are pretty outdated, but that gives you the opportunity to do better. It could at least inspire you and give some idea's :)
And how about
this[
^]?
Good luck!