I'm working on my own tokens (which are just images from movies so they could never be used commercially) and did a quick animation test with a pallet-shifted Mantis for Sprites.
Using the same naming convention I was able to create a green Mantis that had the exact same 3-step L R B T animations, and the game was smart enough to accept them automatically without any alteration to the game's coding on my part.
This means it's comparatively easy to add new player character content to the game, however, adding new
types of animation would definitely require support from Pierre, and I believe it would be an
all or nothing approach. To add idle or attack animations to one sprite, you'd have to add them to
all sprites, which would certainly require a lot of work and backtracking.
If Pierre does implement it, so as not to break other peoples modules developed before the implementation, he'd probably have to tweak it so that if idle or attacking it *looks* for an attack animation frame set, but if it doesn't find it, it defaults to the regular walking animation. I don't actually know if it's possible to do this. My extremely limited experience with XML comes from modding another game,
Dungeons of Dreadmor, which had very similar animation routines, but while it was easy to tell the game to go to X file path and play frames 1-4 with X ms delay between them. Missing even one frame of animation (or more annoyingly, capitalization errors, since XML is case sensitive) would cause the game to instantly crash whenever that animation loop was played.