3D-engine should be a full-blown framework for handling the data model associated to the scene that is to be rendered. Also a compositor framework is to be implemented, which allows creation of multi-threaded pipeline for rendering. Scene system, pipeline and materials should be extendable. Also custom object types should be supported.
3D-engine should also contain Hardware Abstraction Layer to abstract away low-level graphics APIs (such as OpenGL and Direct3D).
Probably only minimalistic audio library in early versions.
Input handling could also be quite minimalistic in early versions.
Library should be supporting at least Windows, Linux, Mac OS X, Android, iOS and Tizen platforms. Windows could support Direct3D and OpenGL in HAL. Other platforms should be supported by OpenGL 3/4 and OpenGL ES (which is a subset of OpenGL 3).
Audio could be supported on most platforms using OpenAL (Windows, Linux, Mac OS X, iOS, Tizen) and OpenSLES (Android).
Window and context creation could be done using GLFW and GLEW libraries on Windows, Linux and Mac OS X. Platform-specific code is required for other platforms.