HDR, if you don't have that. Then I'd start tackling some more complex materials, potentially PBR and more sophisticated specular models.
I'll definately look into that. Any online sources to get started?
I would stay with rendering aspects (vs making it a game engine) for now. Not sure if your shading/lighting includes it, but shadows add a lot to a scene. You might also consider a mesh generator - a cylinder, a sphere, a plane. Quick shapes are very useful for testing if you intend to wrap a game engine around your renderer.
I have all that; should've mentioned it.
My semi-serious reply: you are obviously missing one important feature: gameplay. What's the use of your library if it's never used? Besides, you will have to test if everything works in practice too.
My probably more serious reply: allowing users to specify materials is pretty important nowadays, besides that, I assume all model data can be read from disk already? OpenGL as another back-end would also be nice, since not everyone uses an ugly operating system there are also Linux and Mac users. Some kind of scene manager could also come in handy, just like many other commonly used features of a rendering system. Maybe you should take a look at the Ogre3D feature list for inspiration?
I have a scene structure (with scenes, cameras, lights, nodes) and an offline tool which uses assimp and image libraries to output mesh, textures, etc into a binary file readable by the engine. I do not have any "material" system, though I have heard the term I havn't worked with it nor do I know any details. Any sources for this?