the day they put up a download site with FLACs of soundboards of every show from 1975/04/19 through 1982/11/03 is the day I'll never need anything else from Queen ever again.
To be fair, they usually chose tracklists based on which songs would match each other well enough (in terms of providing enough contrast, consistency, etc). Just like 'A Human Body' didn't make the album because, allegedly, Fred and Brian decided that would've been one ballad too many (and it's, in my opinion, a far better song than at least four that actually made the album).
Chances of finding a rejected song that's as good as 'Teo Torriatte' , 'The Show Must Go On' or 'Don't Stop Me Now' are ridiculously low in my opinion. But chances of finding something (even if unfinished) better than 'Body Language', 'Delilah' or 'My Baby Does Me' (not to mention 'The Call', 'The Kissing Me Song' or almost anything on 'Cosmos Rocks') are probably quite good.
Having said that, considering how quick and comparatively cheap 'Queen II' recordings were, they probably used at least most of what hey had. Albums from 'Races' onwards are far more likely to contain more outtakes and unreleased material.
