In my opinion, we haven't had a real presidential debate for quite some time in the United States. Instead, the parties and campaigns have carefully negotiated formats and rules for coordinated campaign sound bytes and scripted responses. They require that podiums be a certain height, candidates arranged on stage in a particular manner, the questions and topics often be furnished ahead of time, that one candidate be allowed to respond first, and that some topics be avoided altogether. While debates are supposed to be an intellectual argument about policy and ideologies, they are now often mere public performances towards a specific political demographic.
So one reform I would advocate is that you must be polling over 5% nationally on average in order to participate in a debate. This removes those candidates which are now included to make good television and affect ratings as opposed to being serious contenders in the national debate.
Secondly, I would allow the candidates to question other candidates directly. This forces them to think on their feet, and we, as voters, are much more likely to get an honest response as to the candidates views and proposals.
Lastly, I would like to see time limits on candidate answers increased, and time allowed for rebuttals more consistently divided among the candidates. If they do meet the 5% threshold, I think everyone should have an opportunity to speak.
The typical debates we have currently are, in my opinion, not debates. They are PR appearances.