Jimmy Page has stated the band rehearsed a song they never performed before:
There's one number that we rehearsed, I assume that it will make it to the Dome, that we never played at any point in time. It dates from when the band was together between 1968 and 1980. It's a really intense number.He is already on record as stating they rehearsed Houses of the Holy, did they ever perform that song live? If so, Fool in the Rain strikes me as a good choice: it still gets radio play and Robert Plant performed it with Pearl Jam in 2005 at the House of Blues in Chicago.
Perhaps, however (and a lot of websites seem to be interpreting the comments thus) he means a new song, never performed, never recorded.
Three weeks until we have the answer.