Hillary and Trump can never 'debate', in the classical sense of the the word, because neither one of them knows enough about the subject matter.
Hillary has some (i guess) political experience if you count holding a job title, which she only got because of her husband, without ever doing anything of any significance. Trump has some economic experience including global economic experience. But neither of them has consistently worked in government enough to know what the situation really is, or what they could or should do to fix it.
The Hillary/Trump debate would not be a debate. It would be a "SPECTACULAR SPECTACLE!" with all the unknowns in play.
Will Hillary freak-out and start yelling and throwing ashtrays on stage? Will Trump freak-out and start yelling at the moderators and walk off stage. Will Hillary have a stroke, or a coughing fit, or be too drunk to show up? Will Trump hold another fundraiser instead of attending?
The debate would be off the menu in terms of why people would watch it. Who cares what they say? The out of control, completely unpredictable, SHOW is what everyone would tune in for.
Everyone is correct when they say these two are not normal politicians. They are not. They are both clowns in a circus. They are the three stooges minus one. And if they ever meet face to face, everyone would be on edge to see who is going to throw the first pie.