Could you make a simple configurable inventory menu? The inventory API seems powerful, but I have no idea how to use it to create GUI’s without accidentally letting the players get the items that are used as icons.
Something using persistent metadata for entities / players / blocks.
An example of how to use selectors efficiently and powerfully.
Spawning pretty particle effects.
Per player effects, stuff that only a single player can see / vanish / fake blocks
Manipulating the new action bar, wither boss bar, item name bar, and whatever other UI elements that can be abused.
Create a recipe, delete a recipe, modify a recipe.
The obvious answer: unittest.jar, a Unit Test plugin for Sponge! I believe every single program in any language should have one! It simply tests every feature of a program or library. This is especially helpful for diagnostics, but then again you could just make a unit test with jUnit.