I am new to the program, but some of mine are:
Sketch Animation - like Pencil would be very helpful to add small things in some frames and increase Synfig’s usefulness and appeal for all types of animation.
Render Time Approximation - this was a problem for gif mostly(and avi I am sure), but I have found PNG composite is a much better option. I usually just keep the destination folder and hit refresh when I want to check on it.
AVI backgrounds - this would be a nice way to add things to scenes you have already rendered and do not want to edit the source file for. Also, rather interesting for Roger Rabbit type effects.
Particle Tool/Particle Object editor - I also thought it might be interesting to add a Particle tool that is intuitive. Genete’s script is amazing and I am sure i will do alot of things with it once I have a chance to sit down and figure it out, but if there were a way to make it into tool with all the settings on sliders, it would be much easier. His script adds endless possibilities of nice effects to make animations look better. I will register and add it to the wiki page sometime.
As I become more familiar with the program, I will come back here.
Thanks for everything. I love it so far!