Judging by the little to none we know, a complete MtM B-Side (or even the "close to finish ones such as "Chance of Rain") and a discarded ATS song(s) are the closest to "holy grail", besides I want so much to hear OML demos (probably not gonna happen, but had a feeling has some hidden gems on it, especially in acoustic form) and "Breathing Underwater", by the little take from Unshatter movie.
Obviously the chances we get something massive (ha) as Lost are zero to none, but we still can get great tracks, based on the ones that didn't make an album over the years (Slip, all the Meteora demos, Across the Line, Ammosick, Blackbirds, It Goes Through, White Noise, just to name a few)