Spoiler: Not all apps have been written, just like not all books have been written, all meals have been cooked and all words have been invented.
What ails you? What apps do you use today that piss you off? What apps are incomplete or missing from your favorite device (mine being my iPad)? Make a list. Pick the most personal ones and load up Xcode. Trust me, there are thousands of apps that haven’t been written yet. Developer, heal thyself.
The same, of course, is true for every other creative effort.