Assuming you want a practical joke:
1) Request absurd permissions from the developer when they install your app. This could include things like access to their camera, microphone, and location at all times, plus full control of their device's settings.
2) Give the developer a very difficult or impossible coding challenge to complete as a "test" of their skills.
3) Make your app's user interface (UI) as confusing and unusable as possible.
4) Use a lot of useless or unrelated code in your app, making it difficult to understand and debug.
5) Fill up the developer's device with useless data or junk files, making it slow and difficult to use.