I can write this better: Find a prompt made by someone else. And write the same thing better, using the same/lesser wordcount. The compare/contrast possibilities are endless.

See it from my side: Where you write the same main story twice, but from a different person's perspective in the story. Obviously, better if the two side oppose in some way. Another that could be quite fruitful with possible plot lines.

Memento Plotline: Where you have to write your story in non-chronological order, yet it still actually has to make sense and be understandable in the end. Just another challenge that sounds enjoyable. 

And just for general ideas I'd like to see:
Time Travel.
Murder Mystery.
Paradox.

(Also please, please add a minimum and maximum word count rule again and enforce it. One can at least hope right?)