I have two concerns for Trump in the debate(s):
1. He manages to lie and lie, and uses vagueries without anyone effectively calling him out on it and shutting him down.
2. His team will have prepared him to have a few curveballs at least, like actually being able to describe something resembling a couple specific policies. All he would need for his target audience is for them to sound sufficiently plausible and functional on a surface level. People already considering voting for him could hear these things, and be pleased by him sounding more moderate, and feel more confident that he's not so bad after all.
Trump's resurgence has all been about getting people to forget about how absolutely terrible he is, so I feel like all he has to do going in is not say something even R-inclined moderates would have to find disgustingly offensive, and throw out a few policy-sounding things. The pseudo-policies could have little to no basis in reality and have deep flaws just below the surface, but people won't pay much attention to that if it requires going past surface level thinking. Hell, Trump could even say things he has no intention of ever passing, and it would fit right into his established behaviour.