I probably wouldn't have time to do this for next few years, but I wish they'd release regular Notepad as open source. There's a part of me that sort of wants to do some kind of masochistic feature-by-feature port of Notepad into a functional language, just to pay homage to the first editor I ever used.

I know that I could obviously just create my own text editor, but I think it would be kind of fun to specifically recreate notepad of all things.