Great question - It would be a web app so operating systems are off the table. Think along the lines of a project management app, users could log in, create projects, assign tasks, join projects & invite others to projects, assign each other tasks, and track each projects' budget. Something along the lines of that.
I would my link github, but it has nothing worth showing because all of my finished projects have been for work and are not available on github. I usually learn new skills by simply building out the functionality of that skill and then scrapping the toy project and moving on - I don't see a reason to spend time building stuff out I've done in the past and is not going to be used in this project (allowing users to sign up/in for instance). That may not be the best approach, but it allows me to have fun learning new skills and keeps it interesting.