Here are some from me:
1.Adding a import image in one layer that stays for certain period of time then change into a new image rapidly without removing the previous image that has been inserted.
2.When the program crashes, it automatically saves the error message, so that the user can upload the file to the developers, so they can figure out on why it crash.
3.Frame by frame animation with motion and shape tweening in the animation. (very helpful when planing to do a combination with frame by frame and vector tweening)
4.Replace or add GUI codes on windows OS with OpenGL or DirectX (I heard, that you can mix OpenGL with GTK+ from the videos that I found.)
Source code Improvements
1.A new building method on windows and other OS by using a project type software compiler (Visual C++ ,Code blocks, etc for example).
2.A Good documentation on files and folders and a list of svn changes.
3.A TODO list with lots of explanation and detail on each task