This will perhaps come as a bizarre request, but one of the libraries I use frequently is called "The Orchestra Complete" by sonusscore. Its a "lite" library, intended for sketching, with a very robust arpeggiator.
One of the things that I would love to have is a VSL Arp similar to what The Orchestra has, with the ability to see the piano roll / notation used to create the arp.
For those rolling their eyes at this, this has been a great learning tool for me, and all I can say is, take a look at the video, then tell me this isnt cool. For those that already know their way around, something like this could also be handy for quickly throwing in a string section doing a variety of accented 8th notes, or a wind section doing slow trills.
A nice in depth look at the arp of the orchestra... starts about 2m30s in to the video. Note that this video is for the older version of The orchestra. The newer version has about twice the number of presets plus the ability to export the generated arps to use your own orchestral library.
And here is a nice look at some of the presets that come with it (same video as the one above, just later in the video) - starts 6m 30s in to the video above.
The Orchestra Complete Presets
Another aspect of this library is, it does have the ability to import what it creates to use other libraries, but would love to see something like this incorporated into VSL.
I love the idea of user created arps that the community can download, and of course, my hope is that the VSL created product would allow more than 5 instruments (one of the major limitations of "The Orchestra Complete").
And a link to the library itself...
The Orchestra Complete - $399 Euros
The only other company offering anything similar to this is Sonokinetic, I have their Largo and Maximo libraries, but these are not quite as flexible, although, they are much better sounding that the orchestra complete libraries.