Neither Microsoft, Nokia, nor anyone else should fork Android. It’s unforkable.

"If Android were an open platform in the way that Firefox OS or Ubuntu for smartphones were an open platform, the forking suggestion would make more sense."

This comment further down also sheds further light on how difficult it’d be to make such a device:

http://arstechnica.com/information-technology/2014/02/neither-microsoft-nokia-nor-anyone-else-should-fork-android-its-unforkable/?comments=1&post=26198799#comment-26198799