A steeze meter, or an app that tells you if your tee is tall enough.
if you want to get rich make an app/game that is the same as temple run but with a different skin, like the train jumper. this works for any popular app, like doodle jump or draw something / fruitninja ect. really you could just re-skin any popular app and re-release it to get millions of downloads. if you want you could even incorporate skiing like ski run or ski jump or ski something / ski ninja.