In the last two cycles, we have done something different that's worked very well. The first and last presidential debates have been divided into six 15-minute segments. Each of those segments is on a topic that the moderators select and announce roughly 10 days before the debate. The segment starts with a question that is posed to all of the debate participants. They have two minutes to respond, and the balance of the 15 minutes is used for a discussion, with minimal participation by the moderator. In the last two cycles, those have in fact been divided between domestic and foreign topics. That is the way it has worked.
It's not an open-book exam, but it does allow the candidates to understand that these are going to focus on the salient issues and what they are.