I agree it would be helpful to have at least the existing reissues and boxes more readily available for newer fans.
I also agree that the band members appear to be not invested in releasing more unreleased material in general in terms of demos, rehearsals, etc. I think it is unlikely we will see any more of this stuff.
The things I would most value is release of additional live material that was professionally recorded. As mentioned above, a number of shows were recorded for tours after the Lamb. So there are songs from pretty much each tour that were not officially released. I know there is always work involved in cleaning up, mixing, and mastering these recordings before release, so it would have to be financially worth it to do this work.
It goes without saying that I would also love the full footage and performances from the available tours, but this also involves money - acquiring the rights and doing all the cleaning up etc.