You’re referring to mesa-core20. That packages the mesa graphics userspace drivers from the archive for use via the graphics-core20 content interface.
The graphics-core20 meets a slightly different need: it allows us to package alternative (non-mesa) graphics stacks and use them with snaps that consume the interface. While this work was motivated by IoT devices with weird graphics stacks it also “just works” 
on desktop.
What I think you are suggesting is that another snap based on “Mesa next” could be used the same way to provide the latest Mesa userspace. I think that would work. And if the drivers are available from a PPA it would be simple to do.
That seems a sensible thing to do to support snaps consuming this content interface.
Notes
- there are other snaps (e.g.
gnome-3-28-1804) used by “desktop helpers” that also pull in bits of Mesa. That might need someone to do some cleanup to make this generally useful. - It might be done as a track of
mesa-core20(if the Mir team could be persuaded to take on the support)