Hi JT3_Jon,
Trombones are hard to do right even with several choices at hand (at least for me). I admit to approaching them in my pieces with some trepidation. Sometimes I even tend to avoid using them because I feel they will come up short of the goal.
I use a bit of SAM, Sonivox and EW cobbled together with some VSL. It still isn't totally satisfying to me. I'll bet I'm not alone in this. Some of these other libraries also have some pretty good overall low brass - Tuba, Wagner Tuba, etc. Oh yeah, some of these other libraries do have some really ugly brass patches in their collections. They are not a 'go to' answer at all for trombones.
I would think there is a definite opening in the market if VSL would want to move in this direction.
Personally, I don't enjoy spending a whole bunch of time trying to take components from several libraries and pasting them together. If VSL can dig down deep and find the conception and technology to make this work it would be great. I'd love to simplify my brass template.
VSL has exceeded my expectation before (& certainly with the upcoming VE Pro). Perhaps they can do it again.