The First Doctor serial "Marco Polo" is lost. The Apollo 11 EVA video still exists.
In the case of Apollo, we have first-generation copies; it's the equivalent of missing the master tapes (and the chance to make a new first-generation dupe). In the case of Hartnell, all we have is script, some production stills, and fan made off-the-air audio recordings.
Yes, what is next? Bent shadows? Missing rover tracks, that's my guess. But I'll put a couple bucks on "the Earth is too small in the photographs" as well.