There is one big limitation of the current system that has been a thorn in the side of riggers for a long time: the blend weights are stored on the shapekey datablock itself. This makes it impossible to share a mesh with corrective shapekeys between characters, as it is impossible to set different blend weights for different users of the mesh.
If the data model for shapekeys is going to change, it would be great if it were to change in a way that solves this problem. The solution doesn’t necessarily have to be implemented as part of this change, but please keep it in mind when designing a new data model.