Working on some writing that I may or may not ever finish but that I know I'll be working on for many months and possibly a few years.
I'm currently working in (neo) vim 'cause I just use vim for everything and it makes for a great general purpose editor, but...
I'm bit by bit gathering materials (like wordnet dictionaries and such) and looking at the Python curses library and working out in my head what my "perfect" terminal based modal text/markdown editor should be and how I want it to work. No code written yet, just stewing over the "right" interface and mechanisms in my head a lot.
Of course I'm also spending more and more time just working in vim still and doing prose in vim isn't really feeling that weird anymore.