Edit: I just noticed you said you'd keep the basic note/vel mapping. I'll boil my question down to "Does this mean XFade groups would be added as an axis separate from RRs, and will I still be able to use Enable MIDI Selection?"
Original novel:
This looks like a dream come true as someone traumatized by too many huge orchestral libraries done in Kontakt.
I'm curious how you see it interacting with the existing controls. Is the idea that in this mode, samples could only be played back with Sampler.addGroupFilter(), or would assigning a sample a note and rr value still put it in the mapping editor? The xfades would need a way to be edited, were you thinking to move the existing group xfade controls out of the RR groups? Or would it all be managed by script?
The library I'm working on would not have been possible without HISE's "Enable MIDI selection", which let me adjust volume, pitch, start, and end of my legato samples to match them to the sustains. In Kontakt, without combined mics, it's almost impossible to do, though Jasper Blunk masochistically manages it with table modulators and force of will. Spitfire and OT probably do it with their dedicated (proprietary) players. Point is this has always been out of reach for me.
The downside was I needed to use RRs for variants -- vowels in this case. Crossfaded dynamics got split out into separate samplers and controlled with a global modulator.
If "XFade" is added as a native concept, and if it were added as a separate axis, similar to how RRs are now, with a dropdown to filter for a given xfade layer in the mapping editor, that "Enable MIDI selection" workflow would still be possible.
I'm biased, as almost all libraries I've worked on have multiple arts, some with rrs, some with xfaded dynamic levels, some with both. I think supporting keyswitching natively is maybe overkill, as separate samplers for separate articulations is logical and works fine, but maaaaybe it's worth natively supporting crossfaded dynamic levels in the UI?