My initial thought was that if you want to rewrite emacs, why not rewrite it in common lisp? Apparently there are a few cl editors/ides - along the "original emacs" path - but like all things GNU I gather GNU Emacs is more than "just" (a) Free Emacs.
Now, there's a fairly recent (and active) project for implementing a common lisp in c++[clasp] -- perhaps making a real common lisp in rust would make sense?
It's touched on elsewhere in this thread, but one of the benefits of doing the port in the style of remacs is that one has a completely functional editor the whole time: things can get rewritten one small piece at a time because Rust and C have similar binary models. It's easy and fast to call from one language to the other, so one can rewrite a a single C function to Rust and have the rest of the C code call it, and that function call into the rest of the C code (and without any pervasive per-call-site expense).
I have no idea how feasible this is in any implementation of common lisp, but the mere fact the language (AFAIK) requires a runtime beyond the C runtime (garbage collector, etc.) suggests it may not be as easy as with Rust. If it's possible, maybe someone else could start a project attempting that, if they thought they'd enjoy it.
* replace the GNU Emacs runtime written in C/Rust with one which is already provided by a Common Lisp implementation. Each Common Lisp implementation has its own runtime already, but those are not application specific to an editor. Several Common Lisp runtimes for example support multi-threaded Lisp code, native Lisp code, various GCs, support for more data types, better error handling, ... plus they support much of the basic Lisp stuff which is in the GNU Emacs runtime (evaluating code, loading code, ...)
* move some C/Rust code into Common Lisp, since Common Lisp compilers make it possible to write a wider range of code directly in Lisp.
* provide some low-level C code to Common Lisp via it's FFI. That way the C code does not need to be a part of the runtime.
But the topic is dead, because the expertise to do something like that is rare and the GNU Emacs community has no interest moving in this direction.
Now, there's a fairly recent (and active) project for implementing a common lisp in c++[clasp] -- perhaps making a real common lisp in rust would make sense?
[clasp] https://github.com/drmeister/clasp