Well, It'd be an actual pencil test, I imagine. You scrub the timeline and it actually animates. It's not a grease-pencil feature like in Blender or something.
Most of the features would be similar to Flash (as much as I don't like Flash and never use it):
Make new keyframes based on the previous. Extend the exposure of a frame. Onion skin.
Different colors you can choose from do draw lines would be helpful too.
Export the animation with the pencil test, or only the pencil test, so you can know what character parts you need to draw and render out.
I feel like it'd be asking too much of Spine if it also had selection and transform tools too, in the same way you'd lasso a certain area of a layer in Photoshop and CTRL-T, rotate or scale a thing.
I also wouldn't expect the pencil test to be exported into the json+atlas. That'd be wasteful.
The second-best implementation would be the ability to quickly import image sequences of pencil tests that were exported from another program. This has some major disadvantages like not being able to see the proportions of your rig while you pencil test (making them a bit less usable for posing) and just the general slowness involved in moving back and forth between programs.
(By the way, Flash has a nice way of visually adjusting ghosting length. Spriter's UI seems to have adopted it too. They have these little handle things on the timeline next to the current-frame indicator that you can drag in and out to extend or shorten the number of ghosted frames. One for each direction.)
