-PlayerIO
-Spriter
-Spine
These are the ones I am interested in and have looked at doing myself, so if there is no good solution by the time I need these and have time, I'll probably start with these.
PlayerIO and Spine actually seem pretty easy to get started, both have code that is pretty much extension ready, just a matter of translating it for the toolset. Haven't looked into spriter.
edit: Also I agree with above post about the non extension specific thing about folders, but would go one farther and request subfolders within the folders. I like to stay organized and not being able to do something like having a folder for Enemies, then having a folder for each level in that folder is a hassle. Otherwise I end up with too many folders on the dashboard or too many enemies in one folder.