I'm glad this is getting some attention, it's one of a very few reasons that I ever reach for the volume anymore.
Definitely.
That's exciting news. Any implementation based on average gain would be a big improvement for me. What would make it perfect, from my perspective, is if it took into account additional headroom created by internal volume when determining if the average would "blow over." I.e. if the average recommended a -10dB adjustment, but that would cause one track to go over the target by 2dB, volume leveling would apply -12dB when volume was at max, but if internal volume were at, say, 90%, it would just use the estimated average unaltered (-10dB). But I can imagine that might create some other issues (i.e. what happens when someone increases the volume? Probably clip protection kicks on which creates support concerns, etc.).
I seem to recall the issue being that Volume Leveling is calculated and applied before anything else, including the internal volume control. So "clipping protection" has already limited the amount of adjustment before the Volume Control is applied.
What I had previously suggested was that volume adjustments change the target level. So at 0dB the target is -23 LUFS, and at -7dB the target would change to -30 LUFS for example.
One method that might work is to look for an Album Artist(auto) tag with "multiple artists" or "various artists" type values. I don't know how other people use album artist, but JRiver's default logic would populate album artist(auto) with "multiple artists" for a mixed "album" like that. That solution would work for me as I only ever allow album artist to show "multiple artists" when it's a compilation or mixtape, but that might not be an ideal fix for everyone though. Many regular albums have multiple artists on them as well, and if you don't use the album artist tag consistently you could get unexpected behavior; people seem to get hung up on the Album Artist family of tags.
iTunes has a "Part of a compilation" check mark in the info tab. I'm not sure you want to be "like" iTunes though. It is how they've implemented compilations though.
Compilation albums are not "mixtapes".
"Mixtapes" would specifically be if I owned all of a band's studio albums, and then pulled the individual tracks from those to recreate a new "Greatest Hits" collection that just came out.
Because the tracks are from separate albums, they are likely mastered to different volume levels, and would need to be adjusted individually.
This is not something I personally do, but I've seen people request it quite a few times since Volume Leveling was made to be automatic rather than having to manually switch between album/track-based leveling.
Compilation albums would be a "Top 40" album where every track is from a different artist, but the album has already been mastered so that these groups of tracks sound good together.
Many compilation albums crossfade the songs into one another, so you cannot level these individually without completely breaking playback.
"Mixtapes" should use track-based leveling, Compilations should continue to use album-based leveling.
As for marking an album as being a "mixtape" I'm not sure what the best solution would be.
"Album Type" sounds like the right place, but it looks like a calculated field which should probably not be made editable.
A new boolean field might be best, rather than trying to repurpose something else.