You see, this is where I think Trump will actually shine because, generally speaking, debates have something of a protocol to them, and Trump sucks at protocol while Hillary is all seasoned for it, making him the wild card.
How is she supposed to prepare to debate a guy who could feasibily pull a used maxipad from his jacket and accuse Hillary of being on the rag?
When she tries to talk about foreign policy, he just has to bring up Benghazi, or the reset button embarrassment or her vote for the Iraq war. Or, again, pull out a maxipad.
And the reason he can succeed is because she'll finally be the shiny object in front of him, and he can finally focus on her.
Frankly I wouldn't be surprised if he either called her a whore, or a lesbian ("I'm not saying that, but I hear people saying it...") or took a swing at her while telling her to suck his dick.
All totally feasible.
And Hillary could blow a fuse like she did at the Benghazi hearings.