how hard would it be to transpile elisp to common lisp?

Both languages are pretty similar, so it might be a matter of implementing some of the data structures like buffers in common lisp. There is a basic elisp interpreter in one of the cl implementations already. Wouldnt it be nice to get access to cl projects as well as elisp ones?

This has been discussed many times before. Try searching here and more generally.

Elisp can be thought of as an implementation of lisp (lisp-2, if that means anything to you), and in that sense, it’s not hard to get elisp running in various Common Lisp implementations. It can also be thought of as the interface between the emacs editor internals and the user, and in that sense, running elisp in a CL “just” requires implementing most of the editor in CL, in a way compatible with emacs. This is much harder, but has been done now and then, to various degrees. Search the internet for “emacs in Common Lisp” or “emacs hemlock” or “climacs” for examples. So far, nobody has managed to get a working version of enough of emacs in either CL or Scheme to run enough of the most popular/used/etc elisp packages for any of those projects to stick, but there’s continual interest from both sides.