When creating a system abstraction is better to have the platform different APIs hidden by a common interface at the lowest level that makes sense.

Taking in account the different modern (with no fixed function pipeline) native graphics APIs: OpenGLES 2.0+, OpengGL 3.0+, DirectX 10.0+, Xbox DirectX 9, LibGCM

If one was to create a stateless low level graphic API to sit on top of them all, what would be the best way to go to make it as thin and as fast as possible?