I'd like to offer a different perspective on this discussion. It seems this suggestion is offered from the perspective of those who have come to the program after the advent of RealTracks. I have always valued the original midi styles and thought that RealTracks limited the programs capabilities when compared to midi. As good as the RealTracks sound, you are limited to the instruments offered by BIAB.
On the other hand, having grown up with midi and the original BIAB, I have never limited the styles to the description and instruments suggested by BIAB. The beauty of midi is that it is only data, and just because a BIAB midi part is labeled as bass, doesn't mean it is limited to being used as a bass. It works just as well as a bell, chime, pizzicato strings, alien space laser, etc. Between hardware and software, I have literally tens of thousands of midi patches which are useless with RealTracks but work beautifully with the midi styles. However, because of the need to provide a common denominator for showcasing the styles, BIAB is limited to using General Midi patches. Once you break from the GM limitations, these styles lend themselves to the modern sound so often described as lacking in BIAB. Perhaps now that BIAB includes Synthmaster some of these styles can be redone using more interesting sounds.

While I support the original wishlist proposal, I believe it is only useful if you use the instrument as it was original labeled by BIAB. But if you understand that the midi is only data and not instruments, then curating by instrument is of limited value.

As far as user tracks for midi, doesn't BIAB already provide for this with the original sylemaker ? Notes Norton has been doing this and selling his styles since the earliest days of BIAB midi.
There used to be a company called “Twiddly Bits” that sold midi patterns (loops) and I use to use them in the BIAB stylemaker. BIAB recently introduced appegiators into styles with the inclusion of Synthmaster. Decades ago I created a BIAB arpeggiator style using Twiddly Bits arpeggiator patterns in the stylemaker. In addition, the stylemaker is capable of interesting musical output allowing for pattern weighting, macros, and masking based on musical context. All of this became obscured and ignored with the advent of RealTracks.

Here is an example using parts of various BIAB midi styles and the arpeggiator created using the Twiddly Bits midi loops in the BIAB stylemaker:

https://on.soundcloud.com/qQJs1

https://soundcloud.com/cxp-2