Pre-Write the Case Study
I'm taking a fiction-writing class, and last night my teacher gave us this tip: "If you're writing a novel, one exercise to help determine what the novel wants to be is to write a review for it."
Even before you're done, jump ahead and write what you'd like to see in the press once your novel is published. How would they describe it? Not just that it's awesome, but why and what it's about. It's basically a roadmap built on aspiration.
You can do the same thing when thinking about your ad campaign. Write a case study for it. Even before you have it figured out, see if you can write what you'd like the case study to be. What was the problem? Your insight? Your solution?
You'll be able to tell pretty quickly if you're idea's simple and if you actually know what you're trying to achieve. You'll also be able to see if, once all is said and done, you'll actually have a compelling story to tell.