For as much as I like the grand dame of programming languages, I don't think it would be the best substitute for Emacs Lisp. CL is a fairly large language and takes considerably time to learn. An unnecessary burden for someone just wanting to customizes his/her editor. Further, an Emacs in CL obviously requires a CL interpreter/compiler. I'm afraid this limits portability as e.g. SBCL, arguably the best open source CL compiler, supports only a handful of environments. CLISP was pretty portable, but the project seems dead.

I'm sure, if today (or twenty years ago) GNU Emacs would be written, Guile might be chosen as implementation / extension language.

Are we talking about GNU Emacs then? There seem to be a few Emacs or Emacs-like editors in various state of completion / deterioration implemented in CL. Hemlock comes to mind and Climacs.

Guile Scheme would be great for this purpose.