Warning: if you don't subscribe to the One True Religion, this probably won't interest you.
First a bit of introduction. Emacs is an incredibly old and incredibly smart text editor with a smallish C core and most of the intelligent stuff written in a Lisp dialect called Emacs Lisp. Lisp is a very flexible language that intermingles data and code. Making a system built on it extensible is relatively straight-forward. This is probably the main reason behind the success of Emacs, the editor has been extended in literally thousand of directions over the years. If you read my previous blog post, I'm using what nowadays is dubbed aspect-oriented programming to hack a built-in Emacs Lisp module.
It's my impression that even within the Emacs community there are few people who think Emacs Lisp is a great language. It's not Common Lisp, it has some weird idioms and as a small Lisp dialect it still lacks the simplicity of Scheme.
However, weird idioms aside, in my humble opinion the big problem with Emacs Lisp is momentum. When Emacs was started, Lisp was hot. People were thinking that Lisp would be the future. Back in those days, they even had hardware that ran Lisp! The mind boggles. Lisp was an obvious choice for a dynamic extension language.
A die-hard Lisp fan would think the solution is another Lisp, but the official GNU Scheme embeddable language Guile looks pretty dead to me. While learning Lisp is a worthwhile goal that will definitely teach you some lessons, I think there's good reason why most programmers aren't writing parenthesized lists all day long.
I don't think the idea would fly, however, for several reasons. There is an enormous body of Lisp code out there for Emacs. And since Lisp is the extension language for Emacs, everybody contributing to it is a Lisp hacker. It's hard to see how this actually pretty large group of people making up the community would accept anything else than a Lisp. And in the past, even proposals to reform Emacs Lisp or switch to other Lisp dialects have all failed.
The aim would be less code to maintain in Emacs and the benefit of a well-maintained optimizing engine that is getting faster every day, with several independent free ones to choose from.
I think you could maybe sell that idea to the Emacs maintainers. A project like CEDET that replicates the code parsing engines of modern IDEs would definitely benefit from a speedup.
I had a very brief look at the C source of Emacs, and it looks like you would have to rewrite and port maybe 10-20.000 lines of C code. So I think it's doable.