Great work so far, and I have few suggestions that you might take into later versions.

Support multiple project opened in one editor, like that in photoshop. (Sometimes one might need to references skeleton posision or animation from another, and by opening both of them and compare is a good way to go).
Support directly opening up project by double clicking. Now no such method is set and there is no "open with" option on windows, one have to manually open the project inside the editor.
Support pan operation, now only zoom is supported, it works but with pan makes life easier.
Support pixel unit translation using arrow key. Sometimes I work on pixel art and I have strict need to position sprites to an exact location (overlap a pixelated sprite to the template, for example).

Edit:

Added few more suggestions:

Support moving multiple sprites in draw order, currently only one can be moved at a time even choose multiple sprites simultaneously. You can even do so by allowing grouping into a hierachy and move the entire group at once.
Support the delete hotkey. Now I have to manually click delete and button right corner, not so convenient.
Support rename directly in the hierachy instead of popping a window (you could choose to wheather to enable quick renaming in setting). Also support F2 key for quick renaming.
Support refreshing, deleting folders or images in the image folder, in the case of deleting, without actually deleting anything in the operating system.

P.S. The refresing is done automatically but there seems to be a bug where if I change a image, but does not change the name, the change will not be reflected in spine, even I delete the original image, create a new image with the same name, in spine the image with that does not reflect the changes. If name is changed then the change is reflected in spine but if changed the name back, it will go back to the old image (seems something in the cache is not cleared).

Support drag and drop for renaming a image path after missing.
Support redo
Support extension name in image folder (.png etc)
Can we move group hierachy in the skeleton (not in draw order as I mentioned above)? I made a mistake in the hierachy and need to move a group to a different joint but it seems not supported, and to recreate tons of skeleton just to fit the right hierachy is tedious. Seems I can achieve this by using set parents, but a quick drag and drop will be way better. And like quick renaming, I don't think it requires too much of a change in implementation
Support hot key for copying and pasting (ctrl+c/v) in animation mode.
In the drop sheet, add a hot key for selecting every change in a frame (maybe ctrl+A), in this case along with (13), we can duplicate frame and make adjustment easily. Instead of using mouse draging all the way from the top to the button to select everything, which is extremely tedious if repeated a lot.

That's why I can think of now, I will add more suggestions when it came to my mind.

Keep up with the good work!
Thanks!