That Biden will suddenly come down with Covid and be forced to miss the debates. After which, he will have a miraculous recovery and be hailed as a strong and heroic for beating Covid. Either this or some other equally lame reason for Joe to skip out. Or he may simply insist doing the debates remotely from his basement. Cuz of the danger. All the danger.
I personally believe that he will show. The expectations for him are so low, that anything short of a total debacle will be seen as a Biden victory.