Hi there!
I'm quite new to Spine, and have been experimenting with transform constraints (love em!) in a 2.5d rig. It works really well, but I thought I'd mention a couple improvements I think would make it next level.
These are all centred around the idea of creating a master head controller.
1) Bounding boxes for controllers
Maybe the user could place a bounding box, and the controller can not move beyond those bounds.

2) Automatic image replacement according to master controller placement
It would be really cool if you could create big head movements without having to make adjustments - so that the images automatically turn off or replace relative to the positioning of your master controller (defined with the bounding box)

3) This may be a bit of a stretch..but...easy squash/stretch deformers?

I'm pretty sure this must be doable using transform constraints, but I guess I want the easy option 😉
And I guess this goes without saying, but a graph editor for animation!
I also want to say thank you guys so much for the great twitch tutorials by Erika! They are really fantastic, totally broadened my mind about the capabilities of Spine. I spent a whole day watching and absorbing the information like Neo from the Matrix 😉
-Hollie