A couple of years ago an Administrator on QOL said the following about this matter: "Sure enough they've got a shitload of soundboard concerts recorded in stereo onto bug standard quarter inch "reel to reel" tape from the PA feed, but previously to "them", they haven't been able to tinker with the sound too much, and that explains why they've so far left the concept alone.
He could be referring to a LOT of shows where they just have stereo soundboard recordings, and there ARE a lot.
But I really don't think we'll see many, if any, of those ever getting released, outside of some sort of archive live release set, and then probably only for the curiosity value of songs they only played a handful of times."
So, surely there's a nice amount of unheard soundboard recordings in the vault (or Brian's vault). Unfortunately, tragically and sadly it looks like we'll have to wait until after the current Queen bosses are gone.