Emacs

It's always a learning opportunity regardless! Could be a great portfolio project as well. If you're worried about reinventing the wheel: Google every step of the way to find if a well supported package already exists for what you're trying to do. Read code of other popular packages that implement that feature you need and "steal" it from them. Maybe your lisp won't be very idiomatic but that can always be fixed and you have to learn that through experience anyways.