Last year I tried something like this in my undergraduate machine learning class. I don't want to go into details in case I use it again, but it wasn't a brainstorming exercise (I couldn't think of one), but a simple quiz question that introduced part of the material. I had the students vote on the correct answer---and everyone voted wrong, because it was of course a trick question.I’ve found that the very first class period sets the tone for the whole semester. If I don’t get students to participate on day one, they probably won’t participate at all, and the course ends up dreadfully dull. My first lecture in any class thus begins with a brainstorming exercise, in which I get as many different students as possible to voice a suggestion or opinion.
To my delight, I found that year's class asked many more questions than the one before, even though it was significantly larger. This may be due to random variation, or to the fact that I was better at teaching the course the second time, but it's enough that I'll keep trying it.