Ask HN: App ideas to learn Phoenix / Elixir

I've been wanting to learn Phoenix / Elixir for forever basically and have never gotten the chance. This year is the year. Most of my side-projects are small, single-user focused apps but to learn and take advantage of Phoenix I want to focus on something social, collaborative, real-time maybe. Any ideas for something fun HN? Maybe a game? I'll probably write the client for mobile/iOS if that helps but I'm open to anything really.