Is the world moving on from OpenGL, and should SFML 3 follow?
I think at this point an independent graphics backend is the simplest choice for SFML. One which will support webgl, opengl, metal, etc. It needn't implement all these, and could just incorporate a library like bgfx to do all the platform dependent stuff.
I know it might be hard to let go of all the OpenGL code in the library.