If I were a betting man, I would guess "build some new proprietary technology", but they may just go back to WebGL now that they are committing to HTML5.

I really do hope they decide to go with WebGL.

Keeping a fragmented ecosystem with a 'WebDirectX' or some other proprietary new tech will just slow overall progress in this area. For web developers doing 3D it means "This site requires IE/This site doesn't support IE" unless you can support two parallel codebases.