It was weird because we actually had intentionally not prepared any ideas because we were on the "no ideas" track. We spent our prep time practicing questions about ourselves, and our team.
What we learned after and what was said to us by YC staff was that the no ideas track ended up having a higher bar of entry. It makes sense because you're bringing less to the table, so the bar has to be higher.
My suggestion would be, sit down and make some ideas with your team, and build a small prototype. That's really your best bet if you want to get in, it will make your interview more predictable, etc.