The very moment Nikon starts building camera bodies with - optional (!) - Canon EF mount, Canon as a camera maker will cease to exist! Canon would be relegated to third party lens manufacturer status and provider of videocams. :-)
If people can use 8-15, TSE-17/4, TSE-24/2.8, EF 85/1.2, EF 135/2.0, EF 70-200/2.8 L IS II and all the Canon Super-Teles on a Nikon D800, D4, and future D600 ... nobody in their right mind will buy a FF Canon EOS DSLR any longer. :-)
While I really like the thought, it's won't happen before hell freezes over. :-)