Debates are so unfulfilling. Candidates dance around issues, and never have the time to fully respond to attacks. Well, I'd propose 2 changes:

1) Give everyone in the room a button that they can use to decide if you answered the question. If you don't get at least 50% saying yes, you don't get to answer the next question - your opponent gets a clean shot. The incentive, obviously, would be to give a clear and direct answer to the question.

2) Don't limit the time per question. Give each candidate 45 minutes to talk - total - for the entire debate. That lets them make sure that they fully explore whatever issues are most important to them. It's up to them to make sure they don't run out of time before their opponent.

Of course, most campaigns would never agree to these rules.