Writing interesting Prompts

One thing I have been considering as a method to help students find their voice in their writing, is having students create their own writing prompts. I find that when students are able to choose something of interest to them, they are much more willing, and able to create interesting pieces. Typically, I try to give students 5 prompt options on their essays, encouraging choice, but all revolving around theme. (It seems theme is king in our department). However, I feel like this isn't enough. I mean, how often will students have to write theme essays in their real lives? However, how do you teach students to write strong prompts for themselves? I can already imagine telling students they can write their own prompts, and getting blank stares and panicked confusion. Is this something appropriate for high school students? If so, what is the best method for teaching what an interesting writing prompt looks like?