Unreleased tapes?

So I’ve been thinking a lot about Metallica’s next (and likely final) album, and recently saw a vid of their vault, walls filled with tapes and memorabilia. It got me thinking, how much unreleased music do they have on those tapes? Just like with Beyond Magnetic, I’m sure they have some songs that didn’t make the cut for Puppets or Justice, so where are they? Here’s my theory: Metallica have them, and they’re going to unleash a lot of those on their final album. Unreleased riffs, solos, to give a final bang that maybe they’ve been missing.

To go along with this, I’m really hoping they have some cliff tapes they haven’t released, how cool would it be to hear one final cliff part on their final Metallica album?? I mean, even if Rob plays it, the fact that it would be written by Cliff would be insane.