It would be great to have an optional VI/VIM editing mode inside the JSL editor.
Doesn't necessarily need to have all the VIM bells and whistles -- just the command and insert modes, keystrokes and basic operations, and macros.
On a number of occasions I've found myself pasting script code into a GVIM window to manipulate the text, then pasting back to JSL.
I'm sure EMACS users would be interested in a similar editing mode.