Personally, I'd rather toss Lisp entirely and go with Scheme, but something definitely needs to be done about elisp. There's a small group of undergrads here at NU hacking on Edwin, a Scheme based Emacs clone originally developed at MIT. They're (we're, I suppose) porting it to Scheme48.
Those last two would certainly be important as well, but something I have no context on.
- Replace elisp with Common Lisp
- Build os-level threading support
- Build a hardware abstraction layer / target a 'bare' machine.
That gets someone a 'ways' towards a traditional Lisp OS.
I think one of the big questions that arises in for a modern Lisp system is the design of of multiple processes and multiple users.