Pretty strong looking so far, I could see myself futzing about with this now and then.
First thing I'm thinking is some kind of flag system so you can do things like have enemies spawn upon item pickups or create moving platforms and the like.
Other stuff would be a better animation editor and layers, improvements might include stuff like Onion Skinning (an animation tool that renders the next and previous frames as a transparency for easier animation), looping sections, and various kinds of animations like shuffling, looping, and ping-ponging back and forth!