I can speak for our project which uses the plug-in. I hope that 
badlogic can give a general sense for the wider Spine community.
In our project, every 'slot' of the Spine character can be customized in terms of color by the players. We have a MMO-style game in Alpha and customizing your character is a big part of the experience and potential monetization. We can have 30-50 characters on screen at a time right now when we are busy (we are hoping for more in the future, but we will also start 'sharding' on our server-side when it starts to get too big. We already support multiple rooms/areas.)
Each character can customize each slot (e.g. leftArm, head, shield, foot, etc.) with two-color tint, which the players do, to differentiate between each other (we also have different images.) There are roughly 25 slots per character. Currently, we don't do mesh deform, so 25 slots = 25 quads. However, we are considering doing mesh deform and that would probably change it to something 50-75+ quads (dividing up certain quads to have more points so mesh deform will look more appropriate to the design.)
So, we would hope that at least it could be batched per character.
(As an aside, in the current implementation using webGL, we render to C3 texture all the instances of a particular Spine object together using the spine-ts WebGL renderer, they are each rendering to a different dynamic texture though.)
In general, customization is a key part of the game and the two-color tint is a big part of that customization.
Here's a sample screen, it's just one case, we also have mass battles and team battles, CTF, etc. too, but this shows some of the variety of characters and colors.
In our case, we would only need 'new' features for the webGPU renderer as we already have something working for webGL.
As you work on the new renderer, I hope you consider these possibilities for flexibility of future development, allowing vertex attributes, different shaders, and also the possibility of access to multiple textures in a shader (for example we are using that with the current spine WebGL renderer to do palette colors, with a grayscale main texture and a separate palette texture, other uses could be for bump, normal, specular maps.)
Thanks for the discussion Ashley.