I decided to make a game that I wanted to play--not just in general, but for specific occasions. On road trips my wife and I like to pass the time trying to answer trivia questions, and there aren't that many good trivia game apps out there. Even the decent ones lacked features I wanted--so I decided to and created a design document for trivia game I thought sounded pretty fun.
Now, at (somewhat) long last, I'm nearly finished. I have a few features to finish, some interface polish to do, and a few bugs to hunt and kill, but the primary game is essentially finished. In fact, the biggest challenge now is building the database of trivia questions.