We are trying to use gamification (I prefer to call it "Nudging") to get students into Moodle to learn more often. Seeing how schools are so different and each group of students have such diverse interests, I think it's difficult to find gamification that appeals to all learners...especially teenagers! From the research I've done, most gamers are "social", so I think the problem with most gamification attempts is that there is too much competition which is mostly appealing to "achievers". Like you are doing, we also want students to have a say in what the game elements look like, have choices over the "rewards", and provide a learning opportunity for students to create digital art and contribute it to the app. The overall goal for any gamification attempt should be to encourage/reward the behaviors that lead to learning...and of course, make things more fun!