I wonder how difficult it would be to port elisp to Common Lisp and then get GNU Emacs running hosted in SBCL or whatever. I really like the idea of my editor running in the image.
For a terminal editor, it’s impressively usable.
If you just mean writing an implementation of Emacs (or something Emacs-like) in Common Lisp, that's not very hard, and it's been done a few times. See, for example, Lem[1] and Hemlock[2].
Heck, I wrote one for MacOSX in about 2001 or 2002.
If you mean a drop-in replacement for GNU Emacs, that's a lot harder. Besides the UI and the editing infrastructure, you need to write a bug-compatible implementation of GNU's elisp, or you lose the whole GNU Emacs ecosystem. That ecosystem is most of its practical appeal. That's a whole bunch of work.
[1] https://github.com/40ants/lem-pareto/blob/master/lem-pareto-...
I don't think that's exactly what you asked, however, and I agree that a true Emacs running entirely on CL would be preferable.
GNU Emacs has by far the strongest community and ecosystem though.