We will work on improving the Calibre open source e-book reader. (https://calibre-ebook.com/ / https://github.com/kovidgoyal/calibre) This is software used by tens of thousands of people but with a small contributor base, so you'll be making a big impact.
How much experience does your group have? Does the project use anything (art, music, starter kits) you didn't create?
CodeDay Labs team in the advanced track.