My wishlist....
If VSL decides to do a Synchron Brass, it would be interesting to see if they would do it "dimension" style... 4+4+3+1(b. tromb)+1(tuba). Or if they go with 6 horns... or Mahler size of 8. Would they add a piccolo trumpet? Would be neat!
Based on that above video I believe, and I may be wrong, that VSL is normally partial to the dark european sound of the rotary valve trumpets... so it may not be with these instruments.
Off the top of my head, the standard symphonic literature is a high concert D (Bartok concerto for orchestra, Bernstein Kaddish Symphony...). I can say that recording a high D over and over for a sample library would be miserable for most players... sure, there are your 1st call guys, but still not pleasant. But, that would help with those folks who had complaints about the regular Dimension Brass range.
Then, with a brass library, you consider the mutes. I would like to see a handful of articulations with straight mute, and then a handful with harmon mute. Same if they do a piccolo trumpet. The icy orchestration of Petruska's ghost from the Stravinsky ballet calls for muted piccolo trumpet... it's a GREAT effect!
Trombones should have all possible glissandi at different speeds AND fluttered tounged in addition to regular.
For the legatos, there should be pure slur, but then also an option for very light tounge that is used to kind of just bump the air a little bit when landing on the next note. Needs to be a soft bump thought!
On tone, I would like the engineer/producer to really capture the ones that have a more open sound compared to the other brass libraries... more "ohh" in quality. Sometimes, the sound can air on the side of "eee."
Then, a little vibrato control!
Lastly, and I'm not even sure this is possible, but "bells up above the stand" articulations (a couple longs, and a couple shorts)... just for that extra epic Mahlerian blasting. 🎉