If you could write an OS from scratch...

Okay, so most every OS out there is C-based.  If you could magically write a complete hardware-controlling OS all the way up to the UI and applications using a different language, which would you choose?

Though I like Lisp, I think I'd have to choose Erlang, maybe NewSqueak for embedded/smaller systems.

an OS mostly based on a high-level language,
Inferno, colorForth, Singularity, SharpOS, JX, JNode, JavaOS, etc.

Lisp Machine-like functionality in today's hardware,
Modern computers can run Lisp programs a lot faster than Lisp machines.

built-in artificial intelligence services,
Artificial intelligence isn't really useful for very many things. Having it in the core of the operating system seems a little silly to me when most people wouldn't use it, and for those who would it wouldn't be portable to other operating systems.

a virtual filesystem more ambitious than Plan 9
Sure, fuse (http://fuse.sourceforge.net/) + the Plan 9 filesystem is a lot more ambitions than the Plan 9 filesystem by itself.

a grammar-based editor
You can do that on any operating system.