The question is meant to be broad.
I invite y'all to share your ideas here, as they come to you.
Relatedly, if you see a project idea that has already been done, pointing it out as a reply would be useful!
For sharing existing project lists, I suggest doing it in the following post instead: Concrete project lists
Motivation for asking: From now on, I intend to use my answer here to continuously document new ideas I come up with instead of having them logged privately in Google Docs. This is part of my goal of reducing the time between conceiving of an idea and sharing it (How much delay do you generally have between having a good new idea and sharing that idea publicly online?).