So for me it would be: everything that makes Lisp programming easier, like:
- Use a Lisp implementation that's not tied to the editor.
- Slime/Sly + Quicklisp functionality out of the box.
- Integrated syntax awareness, preferably "syntax directed" for programming.
There is more, but it'll be obvious (like updated defaults and looks) unless you want to make an outright clone, or at least a direct replacement option, I wonder if that's what you mean: "rewriting Emacs" seems to suggest it is.
It's difficult. These big old projects like Emacs, and Lisp itself, have both succeeded and failed. There are features that made them last decades and others that made them stop attracting new users. How to capture the appealing parts without reproducing the quirks?