I'm talking with staff about starting a monthly writing contest with a new prompt every month.  I think the key to it depends on what you're aiming for.  How much of the idea you have formed already and how much you're open to changes.

Ex.

https://www.roleplayerguild.com/posts/5210817

If it's a prompt that you have a lot written up already I think it's best to make clear that not everything is up in the air.  If more is open it's good to make that clear and make the prompt relatively succinct.  I favor a word to convey a theme or setting or subject matter and then a couple sentences to make it clear what your limits are.

Writing Prompt : Aspirations

Long term goals, things you wish to be.  Might be nearly in your grasps or something you almost surely will never accomplish but damned if you're not going to try.  Love, wealth, power, fame, freedom, recognition, mastery of a skill or trade.