This has been on my mind for many years, mostly because the rate of updating and feature adding to the existing MIDI plugins in Cubase are very slow - I will say though, I’ve pretty well nearly made entire albums with them as control sources and as rudimentary tools - are still some of the best to this day.
For example, BeatDesigner, could be a much more expanse product maybe via a new version, BeatDesigner2.
But, It would be very cool to see some form of open source/public SDK of VST for MIDI. It would attract more programmers and plugin making enthusiasts to the Steinberg/VST community, and deeper possibilities in regards to programmed music genres, live performance, etc, etc.
With MIDI 2.0 being released, there are likely new potentials from the higher resolution, interfacing protocols, 2-way communication, new supported micro-tonal scales, etc, etc.