I want to hear about your dream project(s). The types of things that you would be excited to wake up and do most days for at least a couple years. Not the specifics, just the context. Something along the lines of:
"I'm interested in the intersection of _____ and _____ and _____......"
In fact, I think it's more fun if you don't have the specifics yet. Leaves room for other people to riff on. Feel free to elaborate on what excites you about these areas. You can definitely have more than one and you don't have to be an expert in any of them.
The only rule is, if you're already working on it, try not to rub it in. Just kidding, there are no rules.