What would be the most damning evidence Wikileaks could use against the US government, as an "insurance policy" against Julian Assange's assassination, and/or Wikileaks being shut down?

My guess: absolute proof that the US government was complicit (or worse) in the 9/11 attacks.

At the very least, I'd be very surprised if there weren't some heavily damaging documents, in some way related to 9/11.

Julian Assange: "However, it is worth noting that in yet-to-be-published parts of the cablegate archive there are indeed references to UFOs."