What could plausibly come out and actually damage Trump?

[Virginiá]

I was going to say a health scare akin to Hillary's, but then I remembered that she is held to a completely different gender-dependent standard and that Trump would rise in the polls substantially if he had a heart-episode or something similar.

He would probably just end up holding a press conference from his hospital room and proclaim that nature is rigged against him. His supporters would then harden their resolve and unite in a bitter fight against reality.