I have actually done some work on a rewrite of the Emacs core in rust[1], so I have thought a lot about this question. Most of the things you would want to change could be fairly easily added to the existing Emacs. The biggest ones that are hard to do with the current core is concurrency and collaborative editing. My particular core is focused on concurrency[2].

Unlike a lot of commenters here, I am trying to stay backwards compatible with elisp. It is not the best, but huge body of existing code is one of the strengths of Emacs. Unlike vimscript, elisp isn’t painful enough to be worth replacing.

[1] https://coredumped.dev/2021/10/21/building-an-emacs-lisp-vm-...

[2] https://coredumped.dev/2022/05/19/a-vision-of-a-multi-thread...